My Bianchi C2C is a 50cm w/ 52.5 top tube c-c, stem 80mm/10deg rise
I am 5'7" with a 32" inseam... short body, long legs.
I have a friend who is my same height and rides a 54cm Trek. (opposite of me long body, short legs) Treks usually have long top tubes. I can hardly reach the bars on her bike, but the stand over is fine.

It's all about comfort. Most women on average have long thigh bones, shorter frames give you more leverage... or so I've been told.