Almost 5'6 and I ride a 49cm bike, just about the exact same measurements as you.
I was originally put on a 50 or 51, don't remember-whatever a small giant is. And I rode 2 of them for a little over a year, I constantly had shoulder issues because I was to stretched out. The next bike I went to(the litespeed) I went down to the 49 and ever since then there is no turning back! I have continously been told by shops, when they look at me, that there is no way I would fit on a 49, but once they see me on it they say it is perfect.(i just have a big drop between saddle and handle bars, which I prefer- but I'm not stretched out).

Sounds like we are similar:-)