I'm 5'6". A 52 is a good place to start for me. I can do smaller with a setback seatpost and longer stem - and depending on the exact sizing/geometry of the bike, sometimes a hair bigger with some adjustments.

I have two bikes right now. One is a 52 with a small amount of setback and a 100mm stem. The other is a 49 with a lot of setback and a 120mm stem. They both fit, although I'm looking for a 73 deg stem (no rise) for the smaller bike to stretch out just a hair more.

Your body proportions, flexibility and to some extent, fitness level will dictate what feels best for you - it's a good idea to try different bikes and to talk to the peeps at the bike shop about what you're feeling.