easiest method to determine frame size is to straddle the bicycle over the top tube, grab the front by the bars and the back by the seat and lift it off of the ground. you should have 2 inches or so before it touches your groin.
5'5", assuming a fairly proportionate torso/legs, maybe something like a 53cm. perhaps a size smaller?
5'11", assuming a faily proportionate torso/legs, around 58-60cm.