"Does this bathing suit come in size 20 1/2?"

Yes, it probably does.

And so do white pleated skirts, ruffled blouses, tiered skirts, obi belts, jeans, horizontal-striped tops and bright colors. All the traditional no-no's of the women's half-size department are now available in sizes 14 1/2 to 26 1/2, due to a kind of large-size revolution over the past few years that has brought fashion and designer names into the half-size closet.

This revolution, obviously, is being welcomed enthusiastically by the 25 million or so women in this country who wear 14 1/2 or larger.

"A whole generation of heavy women grew up in navy, black and brown shapless clothes," says Rena Palumbo of Lane Bryant, which last week hosted with Harper's Bazaar a fashion show for large women. "But there's no reason they can't wear bright colors in a whole range of styles."

And heavier women now can flash trendy labels -- like Givenchy, Yves Saint Laurent, Cacharel, Pierre Cardin, Diane von Furstenberg, Gloria Vanderbilt, Liz Claiborne -- at the thinnest of their friends. A half-size model even paraded this week on a high-fashion Paris runway.

These are several theories about the cause of this "fat liberation."

"Finally, manufacturers are realizing that women aren't all 5 feet 7 and 110 pounds," says Arlyn Blake, senior merchandising editor at Harper's on tour of the U.S. doing half-size shows.

The reason for the new fashion emphasis in this size range, says Blake, is the increased number of professional women, especially older ones, who need working wardrobes, plus the realization by manufacturers that this area is a big (no pun intended) money-maker. "Just because a person is heavy doesn't mean they won't spend money on clothes."

But Harper's Bazaar, chic bible of the svelte, pushing half sizes? It's all part of the new reality, according to Blake. "I find that all women, regardless of size, are serious about clothes and want to look nice and want new ideas."

Lane Bryant always has been serious about presenting new ideas on models who can wear (unaltered) women's-size clothing.

"We never use anyone smaller than size 14 1/2, says Palumbo, whose five models in last week's show ranged from a size 16 1/2 professional to a size 22 1/2 switchboard operator. Because she gets calls every day about modeling, she stresses that the right girth isn't all that's needed.

"You have to be able to know how to walk and turn around and have taste. Some of our models have great legs and big arms, some vice-versa. Just as in real life, you have to learn to capitalize on your best features."

"It's also hard work," says Lula Nicholas, 46 and size 18 1/2, who modeled in last week's show. She has been a free-lance model here for four years, but was "a little nervous" about modeling bathing suits for the first time.

After the show, a number of viewers complained that most of the models were "too thin" and they couldn't really tell how the suits would look on a really heavy woman.

"They need some older ladies who are plump to model," said Angela Deggans, a size 16 1/2 who went up after the show to offer her services.

One staff assistant from a senator's office on Capitol Hill -- a very pulled-together size 20 1/2 -- said that though things have improved tremendously for the larger-size customer, it still is hard to find quality clothing.

"If you're looking for silks or linens it's still difficult," she said. "It's still all those two-piece polyster things in most of the stores in size 20 1/2."

Her shopping trips are often day-long affairs circling large-size stores around the Beltway. Besides Lane Bryant, she suggest Kobiar's in Rockville for good skirts and pants in finer fabrics, Pennington's in White Flint for the best quality and tailoring, and Women's World in Montgomery Mall and Lake Forest. "But it's still hit or miss. Why don't they realize that there are career women out there with money who need good-looking clothes?"

The importance of emphasizing best features in makeup also was pointed out by experts.

"Everybody has cheekbones somewhere," says Emily Pattner, who was doing makeup at the show. "But it takes a knowledge of contouring to bring them out and to slim the face." She suggests investing in a half-hour makeup lesson by a professional.

If this half-size revolution and "shortage" of half-size models sounds too good to be true (half-size models can have their cake and eat it too), and you're ready to put your size 18 1/2 body on the runway, consider this:

"You have to really work at it," says half-size model Nicholas. "Recently I went on a diet, and I was told. 'Don't get too thin, you'll lose your job.'"