When choosing a stability ball you must look at a few factors. Height , weight, and firmness of the ball. Most people go by height only to determine the size ball needed. Keep in mind that the more you weigh the more pressure is placed on the ball and it will create more surface contact, thus creating more stability. If your goal is to engage core muscle (glutes, legs, and abs) then this will defeat the purpose. So remember to factor in all the above mention factor when picking the correct ball for your exercise. Below I have place a chart that can be used as a guide picking out your stability ball.

Height Size
Under 4’8″ 45 cm ball
4’8″ to 5’3″ 55 cm ball
5’4″ to 510″ 65 cm ball
5’10” to 6’4″ 75 cm ball