The comfortable, flattering look of 'one-size-fits-all' clothing

The label says ''one size fits all.'' But does one size really fit all? The answer: It depends. If the dress is cut full, kind of tent-shaped, and has drop shoulders, it's apt to fit a Size 4 to 14.

It'll look OK on a small woman who likes to wear her clothes big or perhaps belted, and it'll be fine on the big woman who likes her clothes loose.

One-size clothing, which began appearing in stores several years ago, has tripled in sales in the past two years. The one-size concept is especially strong with the Japanese designers.

More and more women are enjoying the comfort of these easy styles. They like the freedom from fit problems, and the fact that they can be accessorized so well because of their simplicity.

The steady customers over the years have been the women who are overweight and those whose weight fluctuates.

As a Size 14 secretary says, ''One size is great for me because I'm always dieting. I'm not sure that I'm camouflaging my weight in these dresses but it beats something fitted.''

A Size 4 social worker says, ''I happen to like the big, Japanese look. Naturally, I belt the dress to make it look blousey and to shorten it.''

It's not unusual for mothers and daughters to share the same dress. For instance, there's the Size 14 mother and the Size 6 daughter.

''We have a couple of one-size cotton jersey dresses that we both wear. It's fun,'' said the daughter. ''We get a kick out of changing the look with different jewelry and belts.''

Length isn't a big issue, because just about any length goes these days. The length becomes the decision of the wearer. The 5-foot, 2-inch woman can belt the dress anywhere from a mini-length to a tunic over pants. The 5-foot, 7-inch woman can wear it full length and loose, or belt it. (Many one-size dresses come with a self-belt.)

How a woman wears the dress becomes a matter of personal choice.

''The personality of the wearer is the bottom line,'' explains the manager of a Boston boutique. ''If a woman has a sense of fashion, she can carry off almost any size. It's a matter of confidence. You're always going to find women - regardless of size - who prefer a fluid look over a fitted one.''

When it comes to fabric, gabardines - or something that has a little body to it - are favorites. Cotton jerseys also work because they wrinkle very little and travel well. The best sellers for summer were the 100 percent cottons.

The sizing of the sportswear works much the same as for the dresses. In other words, if the top is wide and has drop shoulders, it should fit most women.

Sportswear in one size includes tops, pants, vests, jackets, skirts, and even jumpsuits.

Tori Ito Bishi, which means ''one size fits all,'' is a fashion house that does garments that are wide, loose, and easy. But the company makes it clear that the apparel is not huge.

The tops usually have drop shoulders, so it doesn't matter where the shoulder lies. The jackets have pads that extend the shoulders an inch or so. This is a plus for the woman with sloping shoulders, because it gives her width at the top. The pants and skirts have elasticized waistbands that stretch from 22 to 34 inches.

Hino and Malee, a husband-and-wife design team, also include one-size items in their line - dresses, tunics, and oversize shirts. Although their collection is geared to the young and trendy, many of their sales are made to mature women who wear a Size 14 or more. The women see this as an alternative way of dressing.

Malee is just a little over 5 feet and weighs 86 pounds. She says she looks best in big, loose clothes. She wears her dresses a couple of inches above the ankles, because she feels long dresses make a woman look taller.

''I'm a Size 2 and I wear a Size 6 or 8 dress. If small or thin people wear small clothes, they don't look so good,'' she says.

''I also think the overweight woman should wear her clothes loose - have room. And if she wants to belt the dress, do it below the waist,'' she says.

Sara Mique Inc. is another fashion house that designs one-size garments. ''We're constantly adding to the line,'' says designer Joan Wieseneck. ''Right now we're working on length to make it more accommodating to women of different heights.''

And there's the Diane Freis line of one-size clothes, with a label that bears her name. The dresses are both one- and two-piece and come in interesting prints.

''I design for real women,'' Diane Freis has been quoted as saying. ''Real women gain and lose weight and they do not always conform to magazine standards of beauty.''