this is the kind of thing that helped me decide (
it's from this site):
Determining what size exercise ball to get
You know what ball you want...but what size?
There are several approaches to sizing exercise balls:
The 90-degree rule: When you are seated on the ball, feet flat on the floor, you want your hips and knees to form a 90 degree angle with one another.
Height: Match the ball to your height - the taller you are, the bigger the exercise ball. Here are some loose guidelines:
Exercise ball sizing
Ball Size User Height
55 cm (21") 4" 11" to 5' 3"
65 cm (25") 5' 4" to 5' 10"
75 cm (29") 5' 11" and up
Sizes based upon height are approximate. If you are near a division (for instance, you stand between 5' 2" and 5' 5" tall) you could opt to go either way. Consider your other body characteristics and how you plan to use the ball.
If you are 5' 3" and weigh 240 pounds, you probably ought to go with a 65 cm ball. On the other hand, if you are 5' 5", but have very short legs, you might prefer a 55 cm ball. Similarly, if you intend to exercise while standing or lying on your back, the smaller ball would work better.
Still undecided? When in doubt, it is safer to choose the larger size. You always have the option of inflating the ball less. Five foot three using a 65 cm ball? No sweat, just fit the exercise ball using the 90 degree rule in sitting. An under-inflated ball will not be as firm, but it will function just fine.
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also, i agree with ishyfishie about how much it's inflated. i use mine as my desk chair, and i have to put more air in it from time to time. i find i like it blown up all the way AND with less air too. i got the 65 cm (i'm 5' 7.5') size, but the 90 degree angle is something i only really get when it's on the deflated end of comfortable.
