Get out your sewing box. Designers in Milan have confirmed that hemlines look best for the moment just below the knee, a trend that started more than six months ago.

The slimming and shortening of clothes, presented this week by some 50 ready-to-wear designers, is hardly an economic measure. Prices are up as much as 20 percent, and by the time the clothes arrive in the stores for next spring and summer they may be even more expensive.

But that won't stop American buyers) who are here, largely because the current crop of Italian clothes, particularly from Missoni and Krizia, are selling very well in the stores. "In a season when sweaters and knits are the most desirable items, you can't do better than these two houses," says Garfinckel's buyer, Joan Carl.

"Our only problem," says Walter Richardson of Neiman-Marcus, "is getting deliveries. If the designers can deliver it, we can sell it." This would seem to bear out the well-known theme that in hard times the well-to-do, unlike you and me, keep on buying, for these include clothes that start at $250 and go up to several thousand.

Italian ready-to-wear sales, in fact, have risen 40 percent over a year ago. And it's not only the export business that's booming. The Italians themselves have started buying again. The consumer strike of two years ago, when high inflation coincided with stagnant low wages, is over.

The Milan show organizers have taken a cue from the French and done them one better in enlarging and centralizing facilities. Last season, the French showed their collections under three tents in the former market area of Paris, now a modern shopping center. Milan has wisely started using three huge exhibition halls at its fairgrounds, which for years have been used for automobile, furniture and just about all other commercial fairs.

With three halls, shows can move from one to another giving designers a chance to clear out of dressing rooms and the huge staff the chance to freshen up the showplace after each event. Unbelievable for this town, most of the shows are starting on time.

So far, there are no shockers in the collections, even the shorter hemlines. And for the ambivalent, there are plenty of up-and-down hankerchief hems. Others can skirt the hemline problem with pants of all lengths.

"Hems are no problem," says Fred McCann, a Sakowitz vice-president. "Women put hems where they want to these days."

Designers are treading carefully in spite of their success, aware that in a tough season economically women often look for the safe and familiar. They have scaled down those exaggerated shoulder pads And they have taken bright colors and used them in color blocks, stripes or scattered about as accents in prints that look like the (early) Russian paintings by Kandinsky.

"Those prints are the real winners at Gianni Versace," says George Philip Kelly, president of Marshall Field.

Shorts popular with Italian designers whenever they think of hot weather, show up more frequently this season, often under split skirts or split dresses.

It has only been since the early 1970s that Milan has become such a center for ready-to-wear. Before that, manufacturers showed in Florence and the big volume of business is still done there.

But it became so crowded in the limited facilities in Florence that in 1972, Walter Albini, wanting to show his five different companies as a group, chose Milan as the logacal place to catch buyers en route to Florence. Little by little, other designers joined him. First Ken Scott, then the Missonis, Krizia and Caumont, and the "new crop" of designers such as Giorgio Armani and Versace.