Originally Posted by
aclinjury
I'd go with a 49 in your case.
I'm 5'7" with 29.5" inseam and all my bikes are XS (~49cm). I slam all my stems and they're all 110-120mm long. I have no reason to think why a 49cm wouldn't fit you.
x2. I'm 5'6" with 29.5" inseam and barely fit a 51cm. You are a bit shorter so the 49cm should fit. Also you might want take into account is the reach will be longer on the 51cm and might not be as comfortable if its way out there. You can make a small frame fit with a longer stem and seat post but harder to make a large frame fit due to the frame being fixed.