When the International Association of Printers and Lithographers comes to town in January to hold a trade show at the new George R. Moscone Convention Center, it won't be a small affair.

There will be 15,000 delegates in all, and they're expected to fill every inch of the center's 275,000-square-foot exhibit hall and 30,000-square-foot ballroom. Between the money participants will pay to rent space and what delegates will spend at local hotels, restaurants and stores, the city figures the show will have an economic impact of more than $10 million.

Luring big conventions and the dollars they bring is what the $124 million Moscone Center is all about, and it's expected to fill a big void in a city where tourism is the No. 1 industry. Until now, conventions have had to book space here in substantially smaller quarters, and San Francisco has lost big shows it otherwise might have had.

"San Francisco is a major convention destination," says Richard Shaff, vice president and general manager of Facilities Management Inc. of California, which manages Moscone Center and the city's two other convention facilities.

"But a lot of conventions have been precluded from coming here because of the lack of space," he adds. "This opens a lot of new horizons for the city."

The San Francisco Convention and Visitors Bureau says tourism accounts for about 60,000 jobs and annually brings in nearly $60 million in taxes, more than half of which comes from a 9.75 percent tax on hotel rooms. (Some of the proceeds from the hotel tax are being used to pay for the Moscone Center.)

Last year, however, tourism in San Francisco took its first nosedive in a decade. An estimated 3.3 million people visited the city, a 6.9 percent decline from 1979 figures. The number of registered convention delegates, meanwhile, fell 13 percent to 646,000.

The overall tourism picture remains troubled because of the sluggish economy, but city officials are confident that Moscone Center will be just the tonic needed to get convention business back on track. It opens Dec. 2 to accommodate 8,000 conventioneers from the American Academy of Dermatology the following Sunday.

George Kirkland, the convention bureau's executive director, says the trade-show industry has been stimulated rather than depressed in the past five years by the country's flagging economy.

"Companies can hit a much larger target audience for much less money," Kirkland says. "How many sales calls can you make in one week in a city? You can probably make five times that number in one good day at a trade show."

Kirkland also notes that conventioneers are usually on corporate expense accounts or can deduct the cost of their trip on tax returns. San Francisco convenioneers last year had an average stay of 4.4 days compared with two or three days for the average tourist, and they spent an average of $519.

"Conventions are as recession-resistant as almost any business can be," contends Roger Boas, San Francisco's chief administrative officer.

Moscone Center, named for the late mayor, won't be a typical convention hall. Except for a glass-enclosed lobby, the building is entirely under ground. The exhibit hall is free of columns and is supported by eight pairs of gigantic concrete arches.

Each arch is reinforced by a network of tension cables running through it and under a seven-foot-thick floor, an earthquake-safety feature of the design. The cables also will help support a huge cinema center, an array of cafes and other attractions that eventually will be built over the center.

Although the San Francisco Redevelopment Agency began work on the design of a new convention facility in 1962, development was stalled by one lawsuit after another. City officials managed to break ground on the project in August 1978, overcame the last legal constraint in May 1979 and are jubilantly watching as the center finally nears completion.

"We've just had to keep the thing on time and on budget, and that's pretty much been a piece of cake," Boas says. "We're only 3.1 percent off budget and about 30 days behind schedule."

Brooks Hall, the city's main convention hall until now, offers only 90,000 square feet of exhibit space. Translated into convention planners' terms, that means there is space for only 400 booths measuring 10 feet by 10 feet. Moscone Center, on the other hand, will be able to accommodate 1,400 such booths.

Because of the size limitations, the San Francisco Convention and Visitors Bureau estimates that the city has been uncompetitive in 15 to 20 percent of the convention market. The loss has been worse in dollar terms, the convention bureau's Kirkland says, because this piece of the market represents the biggest, most lucrative groups.

"We had this much of the market we couldn't appeal to because we didn't have the facilities," Kirkland says, spreading his hands widely to punctuate his point.

Moscone Center already is making a big difference, even though convention planning is a long-term affair and city officials concede that development bottlenecks have prompted a number of organizations to look elsewhere.

Boas says San Francisco will have 39 big conventions next year, a dozen more than it would have had without Moscone, and he notes that this will bring in 124,000 more conventioneers.

By 1985, when the benefits of Moscone are expected to come into full bloom, Kirkland projects that San Francisco's convention business will rise to more than 1 million delegates a year.