Another way to measure the compatibility of your ball to your workout needs is through actual measurement. A small chart is listed below to help enforce this point:
55cm inflates to 21 inches high 5' to 5'7"
65cm inflates to 25 inches high 5'8" to 6'3"
75cm inflates to 29 inches high Taller than 6'3"
As you can see here, depending on the height of the individual we can approximate exactly what size of stability ball would be best suited. This is subject to personal preference however. A beginner may prefer to have a larger ball that is more deflated whereas a more experienced athlete might prefer a slightly smaller ball that is VERY firm! This preference for different sized stability balls brings to surface another common debate among stability ball users, inflation.
The firmer the ball, the more difficult the exercise will be. The softer the ball, the less difficult the exercise will be. A hard stability ball creates a more unstable surface rendering exercise exercise even more difficult as the body's muscles (most particularly the core muscles which include the muscles of the stomach, lower back and obliques) have to work even harder to keep up with the increased demand for balance and stability. On the other hand, a flatter/less inflated ball creates a more stable surface as there is more ball making contact with the ground. Having said that, if you are just starting out, are overweight, an older adult, or you are generally decondiotioned, you may want to consider usung a larger, softer ball. An elite athlete or experienced exerciser would want to have the ball as firm as can be in order to create the moast uneven surface forcing more dependence on the core muscles for stability!
One80.com provides great stability ball exercises customized to its users at no cost.