Even from Interstate 95 South in Virginia you can sense the unavoidable pull of Potomac Mills, like some enormous vacuum cleaner sucking up shoppers into the 115-acre complex of off-price retailers and discount outlets.

Nearing exit 52 at Dale City, the traffic inevitably clots at the sight of the massive sign hanging over the highway for the 1.2 million-square-foot shopping mecca. By 11 a.m., the parking lots look like a prairie of glass and metal as customers hike across the asphalt to one of the many entrances.

Now, after five years, the sprawling, one-story mall will be stretching out even farther, to a mile in length. Recently, mall officials announced that Potomac Mills would nearly double in size by 1992, bringing in about 45 more stores, including a J.C. Penney outlet, to add to the 170 stores already there.

On Sept. 14, Seattle-based Nordstrom, with two stores in the Washington area, is opening the East Coast's first branch of Nordstrom Rack. The store is the hot-retailer-of-the-moment's off-price version of itself, a highly visible catch that retail experts say will entice many shoppers. The Potomac Mills Rack will be one of the retailer's largest at 42,000 square feet.

And a number of new small strip developments -- called "parasite malls" by retail experts -- are springing up all around Potomac Mills, hoping to net some of the shoppers that flock there. Two are already in place and three more are planned, according to county officials.

All this activity comes at a time when consumers, already nervous about an uncertain economy, are looking for bargains. Sometimes jokingly called the "bargain basement of Tysons Corner" by area retailers, Potomac Mills's off-price reputation puts it at an enviable advantage these days.

"We're excited about getting into that marketplace," said Paul Hunter, vice president and general manager of the Nordstrom Rack stores. "With the amount of shopping that we foresee there, we think we are going to get a nice cut and do very well."

In fact, since it opened in 1985, Western Development Corp.'s $100 million investment in eastern Prince William County has been a retailing success for most involved. The mall has generated 3,000 new jobs and almost $4 million in annual tax revenue for the county; annual sales approach $250 million a year.

According to county officials, 63 percent of Potomac Mills shoppers come from outside the area and many travel up to 90 minutes to get there, giving the center a destination status that other retailers envy.

And the quaint historical attractions of the Old Dominion such as Mount Vernon and Monticello have been passed by this symbol of the New Dominion, which is reputed to be the No. 1 tourist attraction in the state, bringing in 250,000 people a week and 2,500 tour busloads a year.

"It's been an important element of development and a very good thing for Prince William County," said Supervisor Kathleen K. Seefeldt (D-Occoquan), where the the retail area is located. "It's clearly one of the biggest destinations."

All that success, of course, has a downside. Many regard the mall as an unmanageable shopping experience even at its present half-mile in length. There are bigger malls in the Washington area, but most are multi-storied, unlike Potomac Mills, which is so spread out that it seems to go on forever. With the new section, the inevitable jokes about it needing its own Zip code, freeway and area code don't seem that far off the mark.

"The good news is that the expansion will give a resounding critical mass and be even more of a magnet for consumers " said Ken Gassman, retail analyst for Wheat First Securities Inc. in Richmond. "The bad news is, if you needed track shoes last time, you'll need combat boots now to navigate those distances."

Gassman points to consumer dissatisfaction in the United States to the hypermarket concept of huge shopping areas where every retailing desire is present in one place.

"If Potomac Mills begins to feel like that, shoppers are going to get frustrated and the mall might be too large for people to shop efficiently and effectively," said Gassman. "At some point, it will be too large and turn on itself."

That problem is high on the list of concerns of mall officials at Potomac Mills. Washington's Western Development Corp. has had experience with size before at its Franklin Mills facility outside Philadelphia at more than 2 million square feet and at its 2 million-square-foot Sawgrass Mills development set to open in Fort Lauderdale, Fla., in October.

"We have discussed a shuttle service and putting in better signs in the parking lot directing shoppers," said Lynn Mitchell, marketing director at Potomac Mills. "The size issue is at the top of our minds."

Another problem is the travel time it takes to get to Potomac Mills, which often feels like five minutes past every driver's breaking point. For some shoppers, it sometimes feels as if flying to New York City is faster than getting there.

The 750,000-square-foot addition, expected to be completed in 1992, will likely bring even more people in, adding more than 1,000 new jobs and potentially millions of customers. Along with the existing outlets of Woodward & Lothrop, Sears and Raleighs, and the planned openings of J.C. Penney and Nordstrom Rack, retail observers are predicting such well-known retailers as Filene's Basement and others to expand to Potomac Mills at some point.

Or around it. In the past year, many other retailers have sought to piggyback on the success of Potomac Mills by cozying up in developments nearby.

The six-month-old Potomac Festival, directly across the street, has a Kiddie City, Marshalls, Circuit City and other stores and is 80 percent leased. "We think we are very well located in an area that has a real regional draw," said Jay Donegan of Trammell Crow, the Dallas-based developer of the center. "We were really thinking of the critical mass of shoppers when we located here."

The $7 million Potomac Mills Home and Auto Center, opened in late 1989, has a W. Bell & Co., Jennifer Convertible and a Firestone Tire store. While the complex looks empty, the leasing agents, Smithy-Braedon, said it is almost full, with only 10,000 of its 63,000 square feet available.

"We think we are offering a range of retail different than the mall, but complementary," said Richard F. Lake, assistant vice president of Smithy Braedon's retail group.

"But a major selling point of the spot is that every car going to the mall is a customer."

The activity has pleased those in the area who remember a different, and much quieter era.

John Gessaman, director of economic development for Prince William County, remembers standing nearly a decade ago on the grassy hill where the popular IKEA furniture store now stands, trying to promote the area as an industrial park for manufacturers.

"I would never have thought it would turn out this way," said Gessaman.

"But it has, and the new expansion is necessary in order to keep pace with competing developments inside and outside the area and, in the end, make this an even bigger attraction."

On a weekday morning just before the end of summer, it already seems like a pretty big attraction as the mall hums with shoppers looking for clothes for the upcoming fall season. "Bigger?" said Evelyn King, a Fairfax resident who has been here all day sniffing out bargains, when told about the plans for Potomac Mills of the future. "Well, great. But I better get in shape."