[header] How to buy an exercise ball [title] Choose an extra-small ball if you are up to 5' (152.4 cm) tall. [step] Exercise balls are available in different sizes, based on the diameter of the ball when fully inflated. The correct ball size is selected based on your height. 
So if you are under 5', the extra-small ball with a diameter of 45 cm (17. 7 in) will probably work best for you.