Royal Caribbean Cruise Line didn't start out to build the largest passenger ship afloat. But as plans progressed, the vessel -- dubbed the Sovereign of the Seas -- ballooned in size to match the majesty of its name.

Some 880 feet long -- almost the length of three football fields -- and capable of carrying 2,690 passengers, the Sovereign was launched on its inaugural cruise last month with former First Family Jimmy and Rosalynn Carter aboard. It will sail the Caribbean year-round on week-long itineraries between Miami and San Juan.

"It wasn't our intent to build one this big," says Mike Petty, the line's director of marketing, referring to the vessel's 14 passenger decks that give it the look of a high-rise apartment building. "But we did $2 million worth of research to find out what the passengers wanted in the way of amenities and activities, and we tried to incorporate as many features as we could." Thus was born the Sovereign, a megaship.

Royal Caribbean isn't the only cruise line thinking big. Carnival Cruise Lines will introduce three new 2,600-passenger ships -- the first, the Fantasy, in 1989 and the other two, still unnamed, to follow in 1990 and 1991. Admiral Cruises expects to launch a 2,000-passenger liner, tentatively named the Future Seas, in 1990.

And a famous Norwegian cruise entrepreneur, Knut Kloster Sr. -- he once headed Norwegian Cruise Line -- is moving forward on plans for a startling 5,200-passenger vessel called the Phoenix World City that could be ready to sail in 1992. Three eight-story resort hotels will rise from its deck, surrounded by swimming pools and palm trees. In weight, it will be larger than the three next-largest cruise ships combined.

As many as 10 new cruise ships of various sizes are expected to be launched in 1988, adding to what the cruise industry generally concedes is a glut in cabin space. As a result, most lines have had to offer a variety of discounts -- among them, substantial savings if you book early -- to attract passengers. Nevertheless, all these cruise lines say they are optimistic about the success of their large ships.

As Petty explains, there is still a huge potential market of Americans who have never taken a cruise. His and other lines are working to get them aboard. In this decade, the number of passengers embarking has been growing at an average rate of 14 percent a year. In 1987, more than 3 million vacationers took a cruise, according to the Cruise Lines International Association, and the figure for 1988 is expected to be 3.5 million. Cruise lines are also interested in tapping the potentially large European and Japanese cruise markets.

The Sovereign and other larger cruise ships are designed for travelers looking for a complete, problem-free resort at sea: lavish meals, health spas, casino, shops, bars, discos, swimming pools and nonstop diversions, most of them organized by the cruise director. Step aboard, and the only decision you face is what activity to sign up for next.

But while some lines are building bigger ships, others, such as Exploration Cruise Lines, feature what in comparison are miniature vessels. Exploration's Majestic Explorer, which cruises Tahiti year-round, carries only 86 passengers. It boasts almost no entertainment facilities; the focus, instead, is educational, appealing to passengers interested in exploring the culture of Tahiti.

Perhaps the antithesis of the Sovereign -- proof of the diversity of cruise options available -- is one of the Maine Windjammers that cruise the New England coast during the summer. These small, sail-powered vessels carry as few as 20 passengers who are expected to help raise the sails, swab the decks, wash the dishes and organize their own diversions. Adventure rather than luxury is the appeal.

Is bigger better?

Petty thinks so. He argues that because of its size the Sovereign, virtually a floating city, can provide passengers with more alternatives than are available on small ships. For example, rather than one giant theater for evening entertainment, the Sovereign has two theaters -- a larger showroom seating 1,050 called the Follies for the ship's extravaganzas and The Music Man, a 680-seater, for less ambitious presentations. Similarly, the ship has two 650-seat dining rooms instead of one with 1,300.

He is fully aware that some potential passengers could be put off by the thought of so many people on one ship, fearing that life aboard might resemble a mob scene. To prevent congestion, he says, the designers have taken several steps to keep the scale of the public areas intimate. In part, this is accomplished by providing the two theaters and two dining rooms, but the Sovereign is offering other innovations.

To reduce the possibility of lines at the purser's desk and elsewhere, for example, an "interactive" video system has been placed in each of the Sovereign's 1,141 cabins. Using an ordinary television screen, the device allows passengers to book shore excursions, check the day's menus, order wine for dinner in advance and keep an eye on their bar tab.

Some travelers might be concerned about the effect so many passengers disembarking at once might have on a port community. Indeed, 2,600 vacationers arriving at once might seem like an invading army to the local residents.

No problem there, says Petty, because the Sovereign will be docking at San Juan in Puerto Rico and at St. Thomas in the Virgin Islands, both destinations that are accustomed to large crowds of cruise passengers. The Sovereign's third port of call is Royal Caribbean's own private beach resort of Labadee on a remote stretch of Haiti's north coast. The inaugural cruise skipped Haiti because it coincided with the country's troubled elections, but a day at Labadee remains on the regular itinerary.

Bernard A. Chabot of Admiral adds that ports such as Nassau in the Bahamas frequently welcome two or more ships at the same time with far more passengers combined than either the Sovereign or his line's Future Seas will carry. He views the Future Seas as "the vacation of tomorrow."

But can any port handle the arrival of 5,200 passengers envisioned for the Phoenix World City? The giant ship will avoid the problem in a unique way, says John S. Rogers, general coordinator of the Phoenix World City Project. Built into the Phoenix will be a marina for four 400-passenger day cruisers. The Phoenix, anchoring near a cluster of Caribbean islands, can send the mini-ships out to several ports on day trips.

Despite the hoopla in Miami last month when the Sovereign got underway, it is not the largest ship afloat today, nor is it the largest passenger ship ever built. Its length of 880 feet is far exceeded by oil tankers, which sometimes are as long as 1,100 feet. And the first Queen Elizabeth, a classic liner of the 1930s, was both longer and heavier. Until this year, the Norwegian Cruise Line's Norway, carrying 1,800 passengers, was considered the largest ship in the cruise fleet.

Until the Phoenix is launched, however, the Sovereign presumably will rule the seas as the world's largest cruise ship. Just a list of its amenities is impressive: 18 elevators, two swimming pools and 1,141 cabins -- all of which can be made up with double beds. No need to sleep on a bunk bed on this ship. Interestingly, the cabins are located at the forward end of the ship and the bars, restaurants and theaters are at the rear. That is so early-to-bed passengers won't be disturbed by the late-nighters.

Royal Caribbean officials are proud of another innovative feature of the Sovereign -- its "Centrum," a five-story atrium "that serves as the hub of the ship's activities." Centrum is a word often used in Europe to refer to a city center, and that is the purpose served by the ship's Centrum -- a meeting place for passengers. It features glass-walled elevators, sweeping stairways, fountains and foliage, a water clock and the ship's many shops.

Just off the Centrum is the intimate champagne bar. It seats 50 people for fine wines, caviar and champagne. High above the rest of the ship, clinging to the funnel stack 12 stories above sea level, is the Viking Crown Lounge. It is a glass-enclosed bar with a 360-degree view of the surrounding sea.

If advance bookings are any indication, the Sovereign seems to be an attraction for the sailing public. "Bookings are going tremendously," says Petty. "They even are exceeding our expectations." The cost of a seven-day round-trip cruise between Miami and San Juan? From $1,390 per person (double occupancy) for an inside cabin to $2,250 per person (double) for a large, deluxe outside cabin. Air fare is included from most U.S. cities.

At the moment, the World City Corp., based in Oslo, is negotiating for a ship builder for the Phoenix. There is a strong possibility that it could be built in the United States, says Rogers, because the weak U.S. dollar means construction costs are cheaper here than in Germany or Japan, two other possible building sites. Estimated construction costs are $750 to $800 million. A French firm built the Sovereign of the Seas.

The concept of the Phoenix is a bit awing to comprehend. It is expected to be nearly a quarter of a mile long and almost 100 yards wide -- "an international city at sea," according to a descriptive brochure.

"You can't really call it a cruise ship," says Rogers. "It's really a floating resort." As a giant resort, he sees the Phoenix attracting large business and professional conventions that no ship today is large enough to handle.

As for the feeling of intimacy? Rogers suggests that because of its size the Phoenix can promise the same kind of privacy a couple might expect when they check into a hotel in Manhattan or Las Vegas. Rather than joining in group activities involving the whole ship, passengers will pursue their own interests among the many activities offered -- much as they would do on a vacation in any city.

Here is what the Phoenix's designers envision: "village squares, parks, 'downtown,' Rendezvous Plaza, Main Street, galleries, museums, a house of worship, tropical gardens, a sports arena, sidewalk cafe's, nightclubs, cinemas, bistros, discotheques, numerous pools, a 100,000-volume library, business center, brokerage office, arts complex and dozens of the world's famous-name shops, boutiques and restaurants."

And somewhere, out there beyond the glass and steel, a sunny blue sea.

ANTEBELLUM RICHMOND: On Feb. 12, Richmond's Valentine Museum unveils a major new exhibit detailing the daily lives of slaves and free blacks in the city before the Civil War. The exhibit runs through Sept. 13.

Entitled "In Bondage and Freedom: Antebellum Black Life in Richmond," the show makes use of such artifacts as ironworkers' tools and a pair of slave stocks; rare photographs and documents of Richmond's pre-Civil War black community, including an autobiography by a Richmond black written while still a slave; video presentations depicting historical episodes and individuals in the city; and the 1812 Wickham-Valentine House (adjacent to the museum), the home of Richmond's wealthiest man in the 19th century.

The intent of "In Bondage and Freedom," according to curators Gregg Kimball and Marie Tyler-McGraw, is to strip away "familiar myths and stereotypes to offer fresh and startling perspectives on free and slave black life in the urbanized South, and relations between the races."

The exhibit demonstrates that free blacks played an "essential role in the city's economy as craftsmen, entrepreneurs, mill and factory workers and servants." As a result, "strict municipal controls over blacks by whites" became "quite lax" in the capital of the Confederacy.

The Valentine, at 1015 East Clay St., is open Monday through Saturday from 10 a.m. to 5 p.m. and Sunday from 1 to 5 p.m. Admission is $2.50 for adults and $2 for children 7 to 12.

Among the other events to be held in conjunction with the exhibit is a play, "Do Lord Remember Me," to be presented Saturday evenings and Sunday afternoons in April. The setting is the "Old Folks Home for the Colored" in a Virginia town of the 1930s. The characters, say museum officials, "share their memories of life as it was before the Civil War."

On March 3 and 4, a symposium will be held to discuss the relationship between free blacks and slaves. On May 8, a bus tour is scheduled to antebellum black houses and work sites in Richmond.

For information and reservations for the special events: (804) 649-0711.

CARIBBEAN UPDATE: Welcome news for last-minute planners. Resort hotels and inns in the Caribbean are reporting space still available for much of February and March, the busiest tourist season on the islands. The reason is not a dropoff in business, says the Caribbean Tourism Association, but "a considerable increase in the number of hotel rooms constructed in the past year."

The association, which represents 26 Caribbean countries, can provide late-comers with information about an island vacation this winter. Contact: Caribbean Tourism Association, 20 East 46th St., New York, N.Y. 10017, (212) 682-0435.

LITERARY ENGLAND: This 13-day package trip is called the "Book and Author Tour of England," coinciding with London's large book fair. It departs Washington on March 26.

Participants will attend a special reception given by the British Crime Writers Association; take a walking tour of Sherlock Holmes' London led by an official of the Sherlock Holmes Society; and explore the places written about by Jane Austen, Thomas Hardy and James Herriott. Among the destinations outside London are Bath, Stratford, York, Oxford and Salisbury.

The price is $1,560 per person (double occupancy), which includes lodging for 12 nights, meals and travel by motorcoach. Air fare to London is additional.

For information: "Book and Author Tour," P.O. Box 31, Upperville, Va. 22176, (703) 592-3755.