I would agree with what everyone has said of just ride the bike and see, personally I ride a 56cm frame, I'm 5'8"/9" and the frame is slightly on the big side vertically but perfect length wise as far as reach, thus I ride with the seat fairly low to accommodate and the bike feels great. However I like a fairly long reach, because of flexibility and what i find comfortable I tuck very low in the drops and this suits me perfectly. However someone else at about 5'8" could find this frame horribly uncomfortable for the same reasons that I like it. On the other hand a rider at your height who prefers a more upright ride could be suited perfectly on the same size frame putting the seat post up if it puts them in a position they like.
Sizing varies greatly between companies and between individual models, I even rode a 58cm bike at a local shop that I found extremely comfortable, but based on calculations I'm probably supposed to be on a 54cm frame. So I tend towards larger frames, on the other hand many prefer slightly smaller frames so it's all preference. Also to put it into perspective a 58cm frame is a bit less than once inch different in seat tube height from a 56cm frame if I understand the measurements correctly.
Finally giant suggest Large frame size(55.5cm-57cm depending on model) for 6'-6'3", so it might be right.