Aside from geometry, which I agree with Knot about, you should also consider the components on the bike, specifically the handlebars and brifters (brake/shift levers). I am not a particularly small woman (5'7") but I find that I vastly prefer narrower handlebars (38-40cm). They aren't necessary women's specific, but are just on the smaller side. Also, people with smaller hands tend to prefer short reach brifters that they can reach more easily. These are things that can be changed once you have the bike (moderately easily) and won't necessarily be on a bike marketed as "women's" or necessarily NOT on a men's bike.