Five menswear designers who are turning their talents to softly tailored women's lines

The fact that strong menswear trends are sweeping through feminine fashions for fall comes as no surprise to those who have been watching the growing influence of menswear designers in the women's field.

The signs are easy to read. The key ones are oversize coats and jackets with softly extended shoulders in herringbones, tweeds, houndstooth checks, glen plaid, and flannel; also big slouchy sweaters topping fuller pants or calf-length skirts. All are worn with fedoras, and are you ready for oxfords? The flat shoe look is important to the overall proportion.

Since tailoring is what menswear designers like Andrew Fezza, Ron Chereskin, Piero Dimitri, Alexander Julian, and Henry Grethel do best, there are more sporty separates than dresses in their collections.

All feature a variation on volume. But to make sure the girls look like girls and not boys, much softness has been added to tailored shapes. Pants, for instance, have more drape and jackets are rounded, sweater-like. Blouson treatments also express the soft, full look. The best fashion in 1984 doesn't fit like a glove.

Underscoring the menswear mood are mixtures of fabric weights and patterns, with lots of deep rich colors balanced by an avalanche of white.

In his premier women's line, Andrew Fezza, a Coty Award nominee this year for his menswear, is using scores of skins - smooth leathers and suedes - and also presenting fabrics of his own design. Included are blends of cashmere and wool with silk, plus novelty silks.

In his second women's collection, Ron Chereskin shows full ankle-length pants and skirts almost that long. They are teamed with tweedy cotton sweaters.

''I worked hard to keep this a tightly edited collection,'' Mr. Chereskin says. ''I think shopping can be confusing if there are too many garments.

''I really had fun,'' he continues, ''designing vests and ties in non-scratchy mohair and wool blends for women. They look tailored and feminine at the same time.''

Basic colors in the line are black and butternut, the latter used both as a solid and for accents. Others are pumice and a rosy red Chereskin calls Indian Apple.

''I'm introducing a few florals for fall,'' he adds. ''They're Matisse-like but actually were inspired by some 1940s costumes the Andrews Sisters wore. For spring 1985 I will be doing many more florals.''

As an alternative to mannish fedoras, Dimitri shows hats with attached scarfs to complement his fall women's fashions.

Everything is elongated ... long belted tops, long tailored jackets, and very long skirts in leather, wool, and cotton flannel.

Dimitri also combines luxury woolens with rain-resistant cottons and leathers embossed with crocodile markings for multi-part outfits.

In developing his women's line (the first one appeared in 1978), Dimitri was inspired by the late Christian Dior's mastery of cut and proportion. An impressive string of accolades, including the Coty Hall of Fame, attests to his success.

Oversized, broad-shouldered silhouettes and full trousers are Henry Grethel's way of transferring mannish tailoring into feminine savvy.

Grethel colors are outstanding, everything from helio pink to heather, spice, and dove gray.

Shirts of crisp cotton stripes, to harmonize with other pieces, have blouson shaping and dolman sleeves.

Under roomy coats and jackets, Alexander Julian favors not only pants and skirts, but a special group of long jumpers and jumpsuits in Italian wool velour. He also features a drape front cuffed pant.

Skirts range from slim to full-pleated and bias-cut circles, all dropping to mid calf. Chemise dresses are in the same length. So is a drop-waist belted dress in jacquard challis.

But whatever your taste in fall fashions, you're sure to like one of the season's accessory tricks because it adapts to almost any look and is a great way to update older clothes.

Instead of wearing one scarf at a time, wear several in assorted patterns, colors, and sizes. Twist long and square ones together, for instance. Combine silk scarfs on a basic dark dress and muffle the neck with woolly ones over coats and warm jackets. You'll even discover that colors you normally wouldn't combine, like rust and purple, can look chic.