If you are a woman whose height registers on the down side of five feet, you understand the meaning of free clothing alterations. If you also happen to be a working woman of this stature, you can appreciate the appearance in Washington of Piaffe, a clothing store that specializes exclusively in petite-sized apparel suitable for work as well as play.

Located on Dupont Circle, the first Piaffe store was born on Madison Avenue in New York, the brainchild of Ellis Oppenheim, a not exceedingly large man himself. After working in merchandising management for Lord & Taylor, Bonwit Teller and the maternity clothing merchandisers Lady Madonna, Oppenheim says he was looking around for a specialized market in which to launch his own business when he opened Piaffe.

"I got the idea because I had been in specialty retailing before at Lord & Taylor, Bonwit's and Lady Madonna, which is a very specialized market," Oppenheim said. "I also happen to have a sister who's very tiny and an aunt who's very tiny. At that point in time I was looking around for a marketing niche and [the small size market] was an untapped niche. So it kind of jelled in my mind, the combination of having worked for Lady Madonna and wanting to do my own thing and the small people in my family."

Oppenheim described his surprise at the reaction in the fashion industry to his requests for women's clothes in petite sizes. Piaffe's stock ranges from size zero, which fits women who weigh 78 to 85 pounds and are 5 feet 2 inches and under, to size 8.

"When we first started asking people to make size 2 clothes, people said 'Why? There aren't that many small women, and they're not that fashionable' . . . Designers tend to get fixed on tall, thin women," he said. "When we first got into the business, nobody was making a proportioned pant for small women. I couldn't believe it. So we made our own, and manufacturers came to us and they are now using our designs . Each season we have taken on a new project, like sleeves on blouses, sleepwear."

While visiting his Washington store last week, Oppenheim disclosed plans to begin franchising Piaffe stores "within the next six months." He said franchisees will be trained at the New York store, although they will probably also visit the Washington location and the second Piaffe establishment in Philadelphia.

Free alterations are absolutely necessary, Oppenheim believes, in a store that specializes in fitting customers who are usually unable to get a good fit. He added that Piaffe sales staff are uniquely qualified to advise customers on how clothes fit because a person must be in the small size range to be hired.

Oppenheim is also a firm believer in the importance of sophisticated clothes to a working woman's professional image, especially if she is small.

"Small women have psychological problems with the world," Oppenheim said. "I remember when I was starting the business, a professor from Yale who was 4 feet 9 and weighed 80 pounds was having problems separating herself from her students because their clothes were similar . And a lot of women lawyers, when they go before a judge, they want to command some respect through what they're wearing.

"One of the rewarding things about this business has been the letters I get from customers. Everything from 'Jesus loves you and so do I' to 'where have you been all my life?' . . . It's been personally rewarding."