Unless you are swimming in your clothes next fall, you won't be in the mainstream of current French fashion.

In fact, the clothes have gotten so voluminous that many of the American buyers here to view next fall's ready-to-wear collections by Paris designers have paid little attention to many shorter hemlines. Some of them are as short as the old miniskirt while others inch up to the knee or just above.

Shortness and fullness are two major themes of the current Paris collections being shown by about 1,000 designers and manufacturers at the Porte de Versailles, the big trade show center here, as well as in couture houses, hotel meeting rooms, restaurants and lofts around the city. Among the other themes are:

Pastel colors about the ione of powdered eye shadow;

Layering of lightweight fabrics;

Clothes contrived to be worn lots of different ways, including inside out;

Big "unlines" and unshaped coats with hugh, cape-like sleeves that wrap up all the big things going on underneath;

Pants of all lengths and shapes, but usually skinny and often tucked into ankle-high boots;

Pierrot collars and Casanova blouses with knickers a trendy theme.

Inflation has touched all the clothes (to say nothing of daily life in Paris, from tax rides to ice-cream sticks). The bigger and bulkier the blouson sweater, the better. Blazers, to look new, are unconstructed and shaped to fit like the jacket of a favorite uncle.

"It's like in the 1950s when your date at a dance would put his tuxedo jacket over your shoulders," said Ellen Saltzman of Saks Fifth Avenue.

Sometimes those big sweaters or blouses are all that is worn over thick tights or pants so tight they are hard to distinguish from tights. It's a look that is already popular on Paris streets.

"We're used to seeing legs covered with pants and now we are looking for tight pants or stockings with thickness to give some of that effect," said Jean Rosenberg of Henri Bendel. "With shorter skirts you can't have spindly legs sticking out, and shorter lengths are coming."

But there isn't any length that you can't find an endorsement for. That holds true for the "Margaret Trudeau dress length" which Marc Bohan, for example, likes with dark stockings and worn for the disco. But never to a state dinner," he added emphatically.

One rule for all the very full clothing is that the fabric must be done in the lightwest weight possible. That is the secret to the softness that typifies the clothing, long or short, and sets it apart from the hard masculine-type tailoring of a year ago.

"Pleated skirts have given way to big skirts softened with shirting. Gabardine has been passed up for jersey. It's all the tricks to make clothes look soft and pretty," said Jerry Solovei of Elizabeth Arden.

But when fabrics are so lightweight, it's difficult to get any warmth, so layering is an essential part of the look. Two years ago Karl Lagerfeld layered one shirt on the other, the bottom one acting as a lining for the others, although it could be worn alone. Last year there were double skirts. This year many things come in pairs, triplets, even sometimes four layers deep.

"If one blazer is good, three are better," says Saks' Saltzman, referring to Armani in Milan who layered three blazers on one model. Castelbajac scaled three unlined hooded coats to fit into one another, to be peeled off like onion skin as the weather changed. But the total effect of the three colors together proved quite remarkable.

Layering is the only sensible way to dress for modern life, says American-born designer Deanne Littell who layered cashmere and silk full dresses with hoods under shawls and over pants much in the mood of clothes worn in the desert.

"We constantly change temperatures from warm to cold in the same day, so our clothes must adapt," she said.

Not all the layering is as simple as Littell's or Castelbajac's. France Andrevie, the current pet of leading stores including Saks Fifth Avenue and Ann Taylor, adds layers to contrast fabric, textures and shapes through rarely mixing colors. Picture this: giant baseball jacket (known in fashion lingo as a blouson) in wool over a loose tunic (hanging below the jacket and vest) over a full skirt (or maybe two) over lacey-knit leg warmers over thick tights, all tucked into ankle-high lact-up hiking boots. The same look is in the collection for evening wear, but the poplin switches to taffeta, the wool to silk, the extra skirt is lace and to pull it all together, lace trim is tied around ankle-high bootlets.

Not only do all of these layers and layers pile on each others, but many are so big and free-form that they can do fancy tricks. Issey Miyake takes the top layer of one costume and ties it around the waist for a blouson jacket, or ties over the shoulder like an inflated shrug or, left alone, works like a coat. A knee-length blouson coat worn unbelted drops to floor length. A sweater-vest unties at the shoulder and becomes a long tunic. And for Miyake's last and best trick, models arrive in silk crept de chineprinted dresses with kites like airbags, in fabric to match, of course, floating behind them. Then voila, the kite converts to a shawl or goes over the head to be worn as a tunic. It would make a grand entrance for the Kennedy Center and in truth, it's not only dramatic but terribly pretty. Kansai Yamamoto also did a version of the same thing.

At castelbajac, his clothes convert, but because his "thing" is functional fashion, everything is made to give another useful purpose. Narrow skirts unzip at the side or more fullness, quilted jumpsuits zip down to shorts then zip off to long jackets then to shorter jackets. Waterproof flaps pull down from zippered shoulder pockets to protect jackets; even boots peel off a waterproof outer layer with fabric boots underneath, good for nice weather. And of course, virtually everything reverses, but you expected that.

It helps, of course if you have friends standing by to help make these changes. Some models had two Castelbajac aides working to zip them into other styles.

To be sure, even if the clothes sometimes appear to be for another age, designers are aware of the recent election which boosted Communist and Socialist candidates in many French cities.

"Police always affect business," says Marc Bohan at Dior. "Now everyone will be afraid to show their wealth. Public parties still give way to more private ones and none will go anyplace in a Rolls-Royce."

Designer Emanuel ungaro agrees. "Fashion is never separated. We have to be less Ostentatious about everything," he said. Deanne Littell is more cautious. "It will affect us but not overnight. If our life changes, our clothing will have to change, but we must wait and see.

But it is not the news that the rumors that are uppermost in the minds of many fashion people here who are wondering about Saint Laurent's health and whether anyone has seen him. In a busload of reporters you can get equal whispers for every side. One French reporter claims to have seen the designer's mother at the couture house, talking calmly, so the assumption is that he must be all right. The word from one couture house is that he's fading fast.

To be sure, a lot of the designers should be worried. At the Porte de Versaille as well as in some designers' salons, the Saint Laurent signature is apparent in the shape, color and even fabrics and trim used. Whatever his own condition, he has made it a healthy season for a lot of others.