Zombie thread. But I still find it interesting.
I'm a tad under 5'6", ride a 53 Cannondale. I wish the bike was a 52 but I think I handle it ok and have a over an inch between me and the top tube when standing flatfooted. Mentally I think I would be happier with a 52. My old mid 80s Asian steel frame is 21 inches, so the 53 is smaller, both have 700s. A friend was trying hard to sell me his older Allez that is 54. It seemed to fit me the same as the 53 and I liked the bike just couldn't see me on a 54 when a 53 is really about the limit.