Bike sizes...top tube, we meet again
I'm 5'3" and my top tube is 510mm. I was measured by my LBS and they said I had a "long torso for my height" (i.e. I have very short legs...) :p
My stem is also shorter than usual (70mm). So I could probably have fit the next size smaller and had a longer stem. Keep in mind that most people can ride 2 sizes, depending on their preference.
If you are proportioned like most women (shorter torso, longer legs), then I would say you are looking for a top tube about the same length as mine :) Remember, you can always get a little longer stem if the bike feels good :) (maybe a 90mm?)
Also, the best way to know if the bike fits you is to ride it ;) :D
Hope this helps!