I'm with Tulip-

I ride with a guy that's 5'4" and has, in his garage, a 48cm, a 52cm, and a 54cm in different brands of road bikes. All fit him fine.
There are also three different people on my old team who ride a size small cervelo TT bike- one is the guy I just mentioned, one is an "average" sized guy, and one is a 5'10" women!
My first bike was a 54cm Javelin that was almost too big, and now I ride a 54cm Blue that fits perfectly. The only difference in the geometry measurements between the two are a degree of difference in head tube angle and 1/2" difference in wheel base.

In other words, every manufacturer will be a little different! Go to a few different shops & try a lot of different ones out. If the shop isn't willing to let you take the one(s) you fall in love with out for an extended test ride of some sort, then look for the ones you love someplace else