It is often called the balance ball, but it certainly won't be if you choose the wrong size and with the wrong size gym ball you could seriously damage yourself. If the stability ball is too small you will not be able to get enough stretch when laying on the ball or if sitting on the ball and lifting weight you will find yourself leaning forward. If the ball is too high you won't be able to keep your feet flat on the floor, will loose balance and end up bouncing around, or worse, rolling off the back of the ball when lifting weights! Having the right sized ball will be safe and your work outs will make sure you get maximum benefit from the exercises.
But looking around the internet at different exercise balls can be very confusing when trying to find out which size you need. Many retailers quote different sizes required for your height. But not only that it can depend upon which exercises you want to do as to which size you go for.
First off, there are usually 3 or 4 sizes to choose from, and more often than not they are colour coded for size, so don't just pick a Gym Ball by it's colour!
The best way is to try one out. If you have a local gym pop in and see if they have stability balls there. Sit on one with your back and legs straight and feet a little apart and flat on the floor. Your hips and knees should be parallel to the floor giving a 90 degree angle.
However, if you are unable to try out a gym ball, then another way to find which size you need is to squat with your back against a wall and lowering down until your knees are at a 90 degree position. It is important to ensure that your thighs are parallel to the floor and not angled down. Mark the wall and measure the height that you need (thanks to GetFitWithVal for that tip). Yet another way is to stand up straight and have somebody measure from the back of your knee to the ground; add 3 cm to allow for depression of the ball; the next size ball after this measurement will be the size you need.
The following is a rough but handy chart to go by for choosing your exercise ball:
45cm Gym Ball - suitable for height 5'0" (152cm) or shorter
55cm Gym Ball - suitable for height 5'1" - 5'6" (155cm - 167cm)
65cm Gym Ball - suitable for height 5'7" - 6'1" (170cm - 185cm)
75cm Gym Ball - suitable for height 6'2" (188cm) or taller
Another thing to think about is how much the gym ball is inflated. The exercise will be more difficult when the ball is inflated more, and thus firmer and less stable. Whilst this is perhaps better for more advanced people, beginners should start with a more deflated ball, which will be more stable and make the exercises easier.
An additional factor to consider is weight. Manufacturers should provide test load figures to the retailer for how much weight the exercise ball will hold.