I have a Rubaix Expert on which I got a great deal. I'm 5'6" and theoretically should have a 52. However, the 54 seemed pretty much OK other than standover wherein the top tube is just about touching me "down there". However, When coming to a stop sign, for example, with both feet on the ground and the bike tilted one way or another, I do of course have plenty of room between top tube and nether region.
While actual riding seemed quite comfortable, I did change the original 100 mm stem to an 80 mm. That made it even better!
I'm now considering another change to a 60 mm stem just to take a little more pressure from my neck. I'll talk to my LBS and get his opinion.
Best regards