DATELINE: DENVER : New Airport Has Its Ups and Downs : Airlines’ uncertain conditions and foot-dragging by United caused plans to shrink and expand.

The new Denver International Airport now under construction on wheat fields northeast of the city keeps shrinking and expanding like the waistline of someone on a yo-yo diet.

The airport will be big when it opens in the fall of 1993, but it is anyone’s guess now just how big. One thing is certain: The 53 square miles of prairie it occupies make it larger than the city of Long Beach and twice as large as Santa Ana.

Although the planned size of the airport’s terminal and concourses has changed several times since construction began two years ago, the land mass alone will change the experience of travelers flying into Denver.

Stapleton has one of the worst on-time records for departures, mostly because foul weather sometimes forces the airport to shut down runways because they are too close together. The new airport’s five initial runways will be far enough apart to allow simultaneous takeoffs and landings during instrument flight conditions, which planners said will significantly cut delays.

In the past, Colorado’s infrequent but famous blizzards not only left airline passengers sleeping on Stapleton’s floors but also created havoc at airports around the country as flights backed up.

“Skiers from places like Los Angeles will be less concerned about flying into Denver because of weather delays,” said Cathy Kruzic of Colorado Ski Country USA.

The size of the airport’s terminal and concourses is tied directly to the unclear future of the nation’s airlines, especially that of Continental Airlines. Continental’s Chapter 11 bankruptcy filing last December made bond-rating companies fidget about its commitment to Denver, driving up interest costs on construction bonds issued by the city.

Continental is the airport’s No. 2 air carrier behind United Airlines. It was United’s reluctance to commit to the new airport that forced Denver to cut the initial size of the terminal and concourses. United is No. 1 at Stapleton, with 185 flights a day, including 20 Southern California flights to Los Angeles International, Ontario, Santa Barbara, Burbank and Orange County.

United balked initially because it questioned the need for a new airport, as well as its cost.

All that changed in June, when United committed to using up to 45 gates at the airport--50% more than at Stapleton. The airline will also add 2,000 reservation jobs in Denver and is considering building a $1-billion maintenance center, which would add 6,000 more jobs.

When United fell in line, so did the financing for the airport, which will cost about $2.3 billion and will be the first new major airport in the United States in nearly 20 years. Secretary of Transportation Samuel K. Skinner, who personally lobbied UAL to settle with Denver, has said the federal government intends to provide $501 million in funding.

Construction bonds sold in April had been rated just above junk-bond levels--at BBB-minus by Standard & Poor’s Corp. A new series of bonds sold after the United commitment were rated A-1 plus by S&P; and Grade One by Moody’s Investors Service.

The once-recalcitrant United now is a big supporter of the airport and aims to keep Denver as its second-largest hub. United accounts for 43% of Denver’s airline business.

“The airport will be a state-of-the-art facility,” UAL spokesman Joe Hopkins said.

United has been touchy about the new airport because it is still talking with the city about its maintenance center. Denver has offered UAL $175 million in incentives, and the state has come up with a tax-reduction package worth about $150 million, to locate the center at the airport.

The state’s tax incentive package is under review by the Colorado Supreme Court to see if it violates constitutional prohibitions against legislation benefiting just one company.

Continental’s financial problems, United’s delays and a decline in traffic because of the Persian Gulf War had forced Denver officials to cut the size of the proposed airport twice, but United’s decision put the original plans back on the table.

The main architectural feature of the new terminal will be a translucent tent-like roof made of Teflon-coated fiberglass suspended by steel cables. The roof will allow sunlight in, which, designers say, will give the building an outdoors feel. And it will emit an eerie glow in the night sky.

Similar roofs have been used at the San Diego Convention Center and Canada Harbour Place in Vancouver, B.C.

Three concourses, with up to 109 gates, will be connected to the terminal by an underground passenger train.

The airport will open with five full-sized runways, with a longer, 16,000-foot international runway completed in 1994. The airport will be able to handle up to 34 million passengers a year when it opens, with expansion to accommodate 110 million by the year 2020.

Stapleton’s peak year was 1986, when 34.65 million passengers passed through it. Travel has fallen steadily since, with last year’s total of 27.4 million passengers making Stapleton the nation’s seventh-busiest airport.