NEW YORK — The sight of all those size-0 models on the Fashion Week runways recalls an oft-quoted line from the film "The Devil Wears Prada." Happy to be a size 6, the impressionable young fashion assistant Andy Sachs is soon brought down to size:

Six, her mentor declares, "is the new 14."

But for the rest of us folks, the question may be even more basic:

What is a 6, and what is a 14?

As any woman who tries on clothes frequently can attest, a 6 in one place can indeed be a 14 somewhere else — or an 8, a 10 or a 2. Which makes you wonder: Is there any logic to sizes, or are they just a random jumble of numbers?

The question might not matter, if the whole issue of size didn't matter. But as the fashion industry has long known, a woman's size certainly does matter — to her. Call it the psychology of size: We care deeply about the number on that tag, even though it's likely no one else will see it, save the person manning the cash register. Perhaps no one else will know, but we know, and that's enough.

Just ask another Andy — Andy Steiner, a mother of two in St. Paul, Minn.

"I hate to admit it," says Steiner, 38, " 'cause I know size is just a number and I like to think I'm too smart — and feminist — to fall for that. But I certainly have a size I consider myself. Of course, I'll buy smaller — and maybe one size bigger. But I'd never buy two sizes bigger. Way too depressing!"

Not just the everyday shopper gets fooled. Suze Yalof Schwartz, executive editor-at-large for Glamour magazine, loves walking into a store and finding she's a size lower. "It can make you feel fantastic," she says. "It's like stepping on a scale. It can make your day. Or, it can ruin your day."

And that feeling, of course, will directly impact whether you make the purchase.

Which is why some clothing lines engage in so-called "vanity sizing" — skewing sizes down to make the customer feel better. It's the reason you might be able to pull an 8 out of your closet from 10 years back, but now, in the same label, you're a 4.

"I can be a very happy 8 at the Gap, but just squeeze into an angry 12 at Club Monaco," says Berett Fisher, a New York mother and creative director. Naturally, she adds, "I don't go to Club Monaco that much anymore."

"Designers know that nobody wants to be a big size," says maternity designer Liz Lange. "Nobody wants to be more than a size 8 or a 10." And she includes herself. "I can't do it," she says of buying a larger size. "I don't want that thing in my closet!"

And yet vanity sizing doesn't explain most of the disparity. The larger picture is that every designer uses his own silhouette, or "fit model," based on his target audience, says Dan Butler of the National Retail Federation.