"American women have great legs. It's time again to show them off in short skirts," declared Bill Blass yesterday morning as he checked the dress racks backstage at the Pierre Hotel before his show.

With that, Blass kicked off the second week of New York showings by joining most Seventh Avenue designers who want women to crop their skirts several inches above the knee for spring.

But unlike others, Blass is not offering an alternative length unless a customer (or store) orders it. For daytime and evening, skirts are short; the only exceptions are long evening dresses and very full evening pants.

Blass has continued his long tunic over slim skirt that his customers, including Nancy Reagan, loved last season, and also showed long jackets worn over slim skirts. While the jackets get some width through padding and shaped sleeves, the biggest change was in his short skirts, which often have a diagonal wrap, a tie, a pleat or button at the hem.

Though these are clothes for hot weather, Blass doesn't make clothes for women to wear to the beach. "My customers don't go to the beach, they just look at it," he said, laughing.

But clearly the Blass ladies go to parties. And he hasn't forgotten that his customers like his big taffeta dresses. But in the spirit of the current collection, those dresses are now very short, sometimes with big tunics over skirts, other times looking like large inflated bags in the middle and caught in at the hem.

Like several Paris designers, he also has a penchant for the skinny and slinky, which he works in matte jersey. His best are the long-fitted evening dresses softened with a big draped scarf anchored to the dress and covering one shoulder.

"Bill's clothes are a good expression of where all clothes are going to be," said Marilyn Kaplan of Neiman-Marcus after the show. "He can even make soft fabrics look architectural." Este'e Lauder, the cosmetics mogul, also praised the range of fabrics Blass used.

One Blass customer undisturbed by Blass' short skirts was Pat Mossbacher, wife of former chief of protocol Emil Mossbacher. "Wherever my skirts are now, that's where they are going to remain," she said.

Washington women, in fact, may share that opinion and wear their skirts where they have been, below the knee.