I just ordered mine from Amazon after going to my first child preparation class. I found on babycenter.com that...
As a general rule:
If you're up to 1.73m (5ft 8in) in height, it's best to get a 65cm ball.
If you're 1.75m (5ft 9in) or taller then it's best to get a 75cm ball.
Since I'm 5'4" I ordered a 65cm ball and it's not specific to pregnancy, just a regular exercise one.