I am 5'1 and just ordered a new frame, a Giant XS (43") non WSD and which will have 700 wheels when its done. It is compact geometry and the more agressive riding position. However, that being said, I have NO torso what so ever and am all leg (34" inseam).

On a Trek, I ride a 50 with a really short stem. Its all about body proportion though. Someone else who is about my height rides a full size bigger than me in the Giant as she is all torso and no leg. I would sit on a few bikes and see where you feel good.