Most exercise ball brands offer specific size recommendations tuned to their specific model. One example is a 38-45 cm ball for individuals smaller than 4'8", 48-55 cm ball between 4'8" and 5'3", 58-65 cm ball between 5'4" and 5'10", 68-75 cm ball between 5'11" and 6'4", and 78-85 cm ball for individuals bigger than 6'4".