how to gauge size of exercise ball?
The rule of thumb is that you want to be able to sit on your ball with your legs at a 90-degree angle or slightly more, but not less. Use these guidelines when you buy a fitness ball: If you are 4'11 - 5'4, get a ball that is 55cm. If you are 5'5 - 5'11, get a ball that is 65cm.