I actually have a few sizes because I like different size stability balls for different workouts. I prefer to use a 55cm ball for PUB abs (pikes, etc), but I use a larger one if I will be sitting or lying on the ball.
I just found a really cool pink 65cm stability ball at TJ Maxx today. I also have an orange 55cm and a red 75cm. I'm a bit of an equipment addict, though. (I even have 45cm stability balls for my kids).
:7
Marla