As we all know, fashion models are thinner than almost everybody. But, it turns out, not only do they have to be "lean and lithe," they absolutely MUST be 5 feet 8 inches tall and have high knees. Yes, high knees. Also, long legs to go with the high knees, broad shoulders and widely spaced eyes.

Barbara Stone, who was in town yesterday plugging Maybelline cosmetics, ran a modeling agency in New York City for 18 years, so she should know what she's talking about. Cybill Shepherd, the girl who gave acting a bad name, and Twiggy, that archetype for anorexia nervosa, and tall, lean, lithe Marisa Berenson were among her charges, so she must know.

But the most important quality, more important even than high knees, she said, is this: The Special Something.

"The special something is something that I'm asked about all the time, and I have never been able to define it," she said earnestly to a television interviewer yesterday. "It's the difference between being just a very pretty girl and being very special and when they have that you know it instantly. It's something in the eyes, in the way they walk. It's the attention they command when they walk across the room...."

Cybill Shepherd had The Special Something. But she needed a little, well, help. "What we did to polish Cybill of course was get off an awful lot of weight, polish a college girl look until she eventually turned into a very beautiful woman. That took a few years," Stone said.

They all need polishing at first. The hair, the makeup, the clothes.The bod -- absolutely no spare flab, folks, not one ounce.Sometimes the nose needs a little work, too, although Barbara Stone said she never told anyone to get a nose job.

The most extreme renovation any of her clients had performed was one undertaken by Veruschka, that 6-foot-plus tiger-maned lady who wore body stockings as street clothes.

"Veruschka had her toes cut off," Barbara Stone said. "I mean she had one joint removed -- most of us have two joints in our toes. I wonder if I could be sued for this? It was because her feet were so big that you couldn't get shoes to fit -- it's a very normal problem. There you are on a set in gorgeous clothes and no one can find shoes to fit your feet..."

Barbara Stone is herself not 5 foot 8, but she is 5 foot 7, and slim. She wore a mauve silk blouse and matching Maybelline nail polish, has short curly hair and perfect makeup. When she was in high school she was a cheerleader and elected Most Likely to Succeed.

She wanted to be an actress, but did not succeed at that, so she got into the fashion business and ended up three years ago deciding she'd had enough of models and all that. "All the world makes jokes about agents. The person who develops someone's career is the agent and they're the last person to be thanked," she said. Now she has her own television production company and last year sold 2 1/2-minute health and beauty programs to television stations that were called "The Contemporary Woman."

Americans spent about $48 billion in 1977 on cosmetics, ladies' clothes and hats and accessories (and that doesn't include shoes), according to estimates provided by the International Ladies Garment Workers Union and the cosmetics industry.

That is more than five times the amount of money the federal government spent that year helping husbandless mothers with dependent children.

"It would take a Margaret Mead to explain it," Stone said. "No, I don't feel guilty. It's not something someone has to do (buy cosmetics). It's no more frivolous than a company selling extra-soft toilet paper. That's not necessary either. People want it. It makes them feel good. And if it makes them feel good, then it's all right. That's just my philosophy. And I've traveled all over the world and I can tell you it's no different anywhere else, except maybe, you know. China. "

Barbara Stone thinks that a woman who does not make the most of herself is afraid of getting attention.

"My philosophy is that a woman who contends that it doesn't matter how she looks and it's what's inside that's important -- although God knows it's far more important, what's inside -- but a person who refuses to do anything and hides behind that as beng intellectually liberated, to me that person is very shy.

"Women who do not go to the trouble to be attractive enough to attract attention -- you don't have to be a raving beauty to attract attention -- are afraid of that attention. It probably goes back to their childhood where they were not boosted by their parents enough. They think if they go around without any makeup and look just sort of gray, people won't see them."

It takes discipline, she said, to keep yourself in line. "I have no patience with ANYBODY who says to me -- short of a thyroid problem -- that they can't do it. I have developed the knack of walking past a doughnut shop and I just feed myself through my eyes."

Barbara Stone is so disciplined that in 1/2 hour every morning she has done 12 minutes of exercise, washed and blow-dried her hair, and applied her makeup.

Not that she believes she is perfect; no one ever does. "I hate my chin," she confessed. "Even Cheryl (Tiegs), she's ravishing and with-it, but if she could change anything she'd probably give herself a fuller upper lip."

Cheryl Tiegs may have a problem there with her upper lip, but fortunately it has not prevented her from earning a gazilion dollars smiling with it. What about the folks who don't look like Cheryl Tiegs, who are maybe even -- as they call them these days -- ethnic looking?

According to Barbara Stone, "the black girl really was in strongly several years ago," in modeling circles, and after a slight decline is now "really back."

"There is an American idea of what a beautiful girl looks like. You can stray a little to the right or left, but you can't go too far. You can stray enough to be black, or Jewish, or Oriental, but you better not go too far afield, you know what I'm saying. To this day I wish I had long, blond silky hair. But who doesn't?"

Barbra Streisand, she said, "has given more boost to the funny-faced woman than, say, a Bette Midler ever will."

But there is hope for one group. "This is really the decade of the older woman," she said. "I know I'm better looking today than I was when I was 30. My figure's better, I'm far more confident, my sex life is better -- with my husband of course. Americans are finally catching up with Europeans in their feelings about older women."

She also thinks that men spend just as much time as women do, if not more, on their appearance, whether thinking about it, exercising, dressing, and so forth.And they are just as obsessed with beauty as women are.

"It's human nature to want to be perfect so that people will love you. It's easier to say to yourself it's got to be because I have thin lips or a big nose that they don't love me, not because I speak before I think or because I'm dull. It's easier to fall on your flaws as a reason for one's insecurities."

At the end of the interview a reporter ventured to have her makeup kit analysed. Spilling out the basic lip gloss, blusher, eye-liner, mascara and foundation in front of Stone like embarrassing secrets, she waited for the word.

"You don't have enough," was the verdict. "Say, why don't we do a makeover on you? We could send someone over from Maybelline this afternoon!"