"I'm 55 years old. I'm a working grandmother. And I'm 4 feet, 10 inches and weigh 99 pounds. Where do I find a dress to fit me?" a desperate caller recently asked one of Hecht's dress buyers.

She's lucky she didn't get a busy signal. There are 14 million "petite" women in the United States (under 5 feet, 2 inches) who have been combing junior and children's departments for clothes for most of their adult lives. The reason: They are too short for regular misses sizes, geared to an "average," 5-foot-6 woman.

Now these forgotten legions have their own size - "misses petites," a category being introduced in many toward working women aged 30-60.

When the Hecht Co., Woodward & Lothrop and Garfnckel's staged a series of misses peltite shows recently, they attracted hundreds of short women, all presumably sick of wearing cutesy pre-teen styles, spending a fortune on alterations or sewing their own clohtes.

"The biggest problem is that you can't just run in and buy something and wear it right off the rack," says Pat O'Connell, who is 5 feet, 2 inches and 115 pounds.

Anne Cross, a Washington grandmother who's 5 feet 1 inch, says she has had problems finding clothes since her younger days. "In my age group, the styles are the problem ' I'm too old for funky teen-age things, yet the regular misses sizes are all way too long."

But with much the same fanfare manufactures used to court half-size customers by last year, about a dozen New York dress firms have come out with missess petes lines for spring.

"Those poor little things," says Benard Wolfson, president of a new misses Petite company (called Route One) and father of 5-foot actress Madeline Kahn (who, he says, always has a problem buying clothes.) "They haven't been able to find a dress they could put on without spending lots of money on alterations because every part of the garment had to be taken apart."

Like most misses petite lines, the 45-piece Route One collection is priced from $30 to $60, a range geared for what manufactures call the "older career woman." The fabrics are predominantly easy-care synthetics or blends.

The collections, which are all predominantly one or two-piece dresses, were prompted partly by greater demand for dresses in general over the past fer seasons. Yet dresses posed the biggest dilemma for short women, since many recent popular styles have included border prints, tiers, flounces and pleats, all of which are very hard to alter without ruining the line of the dress. And it is dresses that most of these women prefer to wear to work.

"I want something not too wild, but not really old ladyish looking," says Rosie Miller, a 34-year-old bookkeeper from Silver Spring. She is 5 feet 1 inch and 115 pounds, and very happy with the new size 10 petite.

Manufacturers know this and their designs for petites tend toward teh conservative, with few girlish ruffles of low necklenes and with lengths that fall a few inches below a short knee.

Sketched from left: Oatmeal-colored polyester/flax drawstring waist dress with matching vest by Newton Centre.

Soft, knit jersey dress with peplum (Woodward and Lothrop, $44.) by Petites by Verona. (Garfinckel's, $42.)

Matthew Love's print blouson dress of a wool-blend fabric. (Hecht's, $40.)

A two piece outfit with embroidered collar and pockets of a polyester/avril fabric made to look like handkerchief linen by Route One.;Hecht's $45.)