Sarah-
I'm 5'2" and have a 55cm one. I got one from Spri but it never "worked" right for me. I got a versaball pro about a month ago, and an air compressor, and I like stability ball work a lot better now.
I went through Cathe's buyer's guide. After the Road Trip people came back and were posting about how much easier pikes, etc seemed with the balls at Cathe's gym. That's why I got the versaball.
HTH