The retail world is flooded by a sea of clothing sizes. To find the right one, women must tread through “misses,” “petites,” “pluses,” “juniors” and more, depending upon the store. And even after discovering a favored fit in one brand, the same woman may balloon to a larger size or slip effortlessly into a skinny-minnie one by another designer.

In many cases, this seesaw in sizes has little to do with the progress or failure of a New Year’s weight-loss resolution. Instead, the culprit is vanity sizing, or the fact that the U.S. government doesn’t enforce a standard of measurements for women’s clothing sizes and, therefore, designers are free to come up with their own.

A standard measurement chart does exist, but it may be next to impossible for the government to require designers to stick to it, says Paula Minydzak, an adjunct instructor in fashion for the Art Institute of Pittsburgh, who also is a tailor at a Men’s Wearhouse.

Standardizing sizes can complicate transactions between domestic and foreign designers whose countries sometimes have different measurement charts, she says. It also may turn some women off from shopping at first if they wear a larger size under the government-regulated measurement chart.

“It’s the equivalent of sticker shock,” Minydzak says.

So for now, the size guessing continues, which has its pros and cons.

“We have lots of different-shaped people, so if we have lots of different-shaped garments we can fit everybody,” says Tammy Kinley, associate professor of merchandising at the University of North Texas.

For designers, the flexibility to come up with their own sizes helps them create and market clothes for their target consumers – and their egos.

“It’s easier for someone to sell clothes when they make people feel good,” Minydzak says. “One of the best ways to do it is make a woman feel like she wears a smaller size.”

But toying with people’s perceptions of their body image can have consequences. In 2005, Kinley conducted a study in which 75 women between the ages of 19 and 58 were instructed to try on different sizes.

“What I found was a strong age relationship,” she says, adding that women in their 20s seemed to care more about fitting into a smaller size than those in their 40s or older.

In another study, from 2003, Kinley and some students measured about 1,000 pairs of pants by different designers of similar styles and cuts and found “statistically significant differences within some size numbers,” she says. She also observed that clothes by designers with higher price points tended to be more generously cut, with Ralph Lauren and Calvin Klein among the most forgiving.

Minydzak, who makes patterns for her own designs, has noticed that clothes by more affordable brands tend to run more narrow in the sleeve and leg.

Many brands view their sizes not as mind games but as ways to better relate to shoppers’ needs, wants and body types.

“American Eagle Outfitters chooses fit models that reflect its target customer base of 15- to 25-year-olds. We also focus on making our sizing consistent within our own brand,” according to an email from a spokeswoman for the company.

For clothier Anthropologie, the goal is to sell quality clothes with consistent sizes that appeal to the body shapes and lifestyles of its target customer base of 28- to 45-year-olds, says Wendy Wurtzburger, chief merchandising and design officer. The company tracks its consumers’ likes and dislikes by studying reviews of its merchandise and learning about other places they shop.

“Our intention is to have a size grade for her that is flattering but is honest and isn’t trying to make her feel like something she isn’t,” Wurtzburger says. “We hope we make some beautiful apparel she’ll really love with great details, and that’s why she’ll come to us.”

Other stores have disregarded the typical double-digit numerical-size chart altogether. Sizes used by the clothing chain Chico’s range from 000 to 4.5, with the former being for women who usually wear a zero and the latter for a size 22.

The tendency to skew sizes downward is a more recent occurrence, says Richard Parsakian, who owns Eons, a vintage clothing shop in Pittsburgh. When women come to his store, they often are taken aback by the variances in sizes between the clothes of today and yesteryear.

Someone with a 24- to 25-inch waist may wear a zero now but probably would have been sized as a 10 or a 12 in the 1950s, he says.

And what changes lie ahead are anyone’s call. Academic studies have found that the public wants clothing labels with more sizing information on them, Kinley says. Stores such as Macy’s and Lane Bryant recently have launched Web components with tips on what sizes, brands and styles may be best suited for their customers.

But whatever designers come up with next, comfort and satisfaction – not size – should be the motivation behind a purchase.

“We have to use sizes as a guide but respect the fact that that brand does not mean you are that size,” Minydzak says. “You are your size. Your body is your body, and there is no brand or no standard set of measurements that is going to be made exactly for you as a woman.”