stability ball core exercises. For most exercises, though, it’s best to have a correct sized equipment. Most balls come to the market in three diameters based on our height: 55 cm for those between 4 ft 11 inch and 5 ft 4 inch, 65 cm for those people between 5 ft 4 inch and 5 ft 7 inch and 75 cm for tall people between 5 ft 11 inch and 6 ft 7 inch. A good rule of thumb for finding the right fit is Sit on the ball and make sure our hips and knees are at right angles with the floor.