measure your sitbones in mm. there are ranges that will be most comfortable for your width and your style of riding.

here's the guideline from the "butt-o-meter" (yes, I have one because I use it for bike fits). the range on the narrow end is intended for those who ride in a more aero position (ie riding forward on the soft tissue), while the range on the wide end is intended for those who ride in a more upright position (ie sitting on their sit bones).

SIT BONE WIDTH -- SADDLE WIDTH RANGE

130-160mm -- 143mm -- 155mm and wider
100-130mm -- 143mm -- 155mm
70-100mm -- 130mm -- 143mm

I've been fitting women (and men) on Specialized saddles for over a year now. I have a saddle demo program with my bike fits. I'd say 60% of the women I fit on a 143mm saddle, 30% on a 155mm and 10% on a 130mm.