Fitzie, that is a tough one because it depends on the length of the torso, the leg lenght, etc. Although at 5'3" I ride a 49cm road bike--but I may be a 48cm in a road bike made by another manufactuer because the length of the top tube is different depending on the builder. Each bicycle manufacturer uses different frame geomentry. The best bet is to check out a few different lbs in your area, get properly fitted and test ride.
Hope this helps.