What is the circumference? Does it need to weigh a certain amount too? I was doing the Timesaver #2 workout last night that included scenes from her "Supersets" workout. I got a cheapie ball, but the one Cathe and her crew uses is MUCH bigger than mine. I checked the box on the one I got to see how big it is, but it doesn't say. Also when I try to follow her and do the exercises on the ball like she does, my body is so much lower to the ground than when she does the exercise, so I figure my ball is much smaller than hers is, and she and I are just about the same height (I'm 5'3").