The big message from the Paris designers is: Shape up. If under your big tent dresses you've been concealing a tiny waistline, a taut torse and small derriere, you're in luck.The shape for the spring is hourglass, and the new clothes will give you plenty of chance to show it off.

Starting at the top, if you thought padded shoulders would pass with the football season, you are wrong. There are various degrees of padding, but clearly the broad-shouldered look has a wide following. Then here's how the rest shapes up:

Defined waistlines. Clinched, belted, wrapped or pulled in with cummerbund, the important thing is that everything has a strongly marked waistline - the smaller the better.

Shorter skirts. The new proportion demands a hemline cut off an inch, sometimes two, below the knee. Some designers are showing them longer, but it is now obvious that the shorter skirt is the coming thing.

Bare shoulders. The strapless look is back, and spaghetti straps and one-shoulder tops are showing up all over.

Bright colors. Most of the collections have more than one, always in a clearly defined space: Blocks of color or piping. Plenty of white, too.

Nautical themes. The message comes through loud and clear with middy tops, stripes, brass buttons on navy, bog hats and enlisted man's pants. The look is everywhere but where it all began - at Chanel.

Shiny textures. Glitter for day: with lurex striped T-shirts, iridescent fabrics, metallic belts, satin shoes, lacquered straw hats.

And to ensure the hourglass look, several designers have revived the peplum, a flounce of ruffle around the waist. At Chloe, where the models have tiny hips, Karl Lagefeld has even padded the peplums.

Clearly these are not clothes for the shy or retiring. While the shapes are usually pure and simple, by the time they are gussied up with accessories, they are head-turners. The big plastic cutout pin has succeeded junk jewelry; and huge earrings, shiny fabrics, dayglo colors and even stockings with seams scream, "Look at me."

There is a bare look apparent-in tube tops and bandeaus under jackets, and there are few shirts anywhere. "A breast or two," says Geraldine Stutz, president of Henri Bendel, "helps the look." And Kal Ruttenstein of Bloomingdale's is sure he spotted a falsie in one of the collections, maybe at the Chloe show where Karl Lagerfeld's strapless tops were boned and lined with fiberfill.

"I think by next summer the popular look will be a tube top and pedal-pushers," Stutz predicts.

Pedal-pushers, capri pants whatever you want to call the trousers cut off below the knee - are everywhere. Sometimes they hide behind panels as at Sonia Rykiel, or sometimes under shirts and dresses, as at France Andrevie, Thierry Mugler and elsewhere. But at othertimes they are just out there by themselves, as at Yves Saint Laurent, among others.

There are bermuda shorts and short shorts (sure to show up in discos). But generally, if you shortened your trousers to ankle length and tapered them a bit, you'd be in business. Of course Yves Saint Laurent - not wishing to offent his faithful who prefer the shoe-top variety - has included those, too.

For those wavering between the long and short hemlines, there are plenty of uneven ones: Kenzo by day, Valentino by night, as well as many others, and they are both sexy and add mobility.

"The shorter length cupped under the fanny is certainly not for everyone," says Marjorie Schlesinger Deane, president of Tobe and Associates, consultants to stores including Hecht's and Woodies. "Many are showing longer lengths, Emmanuelle Mhanh among them, and they look very pretty."

"By next fall," predicts Ruttenstein, the mid-calf skirt will not look fashionable.

What everyone agrees will be fashionable are the strapless and bare looks. (Hanne Merriman of Garfinckel's has already had proof. The strapless dresses in their windows last week by Sybil Connolly were the first to sell of the Irish imports in the store.)

See-through fabrics, such as the organdies at Kenzo and the chiffons elsewhere, are likely to show up in Washington stores with camisoles and slips to be worn underneath, according to Janet Wallach of Garfinckel's.

Designers agree that the reason for the slimming down and shaping up is that the big look has been a bust. "Women have gotten too relaxed in the big shape," says Saint Laurent. "It's just been too messy," says Lagerfeld. And in Washington last week, Bill Blass was saying, "Husbands hate them."

"Women have just passed up the big look," says Stutz. Business is tough enough, she says, with customers reluctant to release big bucks for designer clothes, particularly if the look is not terribly appealing.

Ruttenstein has found that prices affected by inflation and te franc-to-dollar ratio - are now at least 10 percents above a year ago. "I don't know what we will end up buying," he says. "But I know we will have to pass up some things, because even for the most fashion-conscious woman, the price will be too high."

Paris wholesale prices are tripled or quadruped by the time they get to stores after duty, shipping and other costs. "And now that we are getting four francs to the dollar rather than five, like a year ago, the prices in the end are really much higher."

The prices may seem especially steep because the looks all seem like you've been there before - in the 1950s. But, of course, now there's a difference. "Women were shorter and stouter then," says Bernie Ozer of Associated Merchandising Corp. The buying office for over 30 department stores including Woodward & Lothrop and Bloomies in Washington. "The model size back then was 10 or 12. Today models are a taller and skinnier size 6 or 8. And Ida Lupino and Betty Davis were really very short. Today's actresses are much taller.

"If my mother saw these clothes, she would consider them quite matronly and dated," says Ozer, who finds them quite sexy and modern.

Stockings with seams are modern? "I guess if you have never gone through the business of trying to keep them straight, it seems like an amusing idea," says Gerry Stutz. "I really hate to start that again," she conceded. "But I will."