To get the most out of your exercise ball, you need to choose a size that is suitable for your height. Swiss balls come in different diameters and should be chosen depending on your weight and height. Ideally, the hips, knees and ankles should all be positioned at 90 degrees to each other when you're sat down. But if you cannot test the gym ball before buying, see below for a guide below from Spine Health. Diameter Person's Height Diameter: 45cm for heights: 5ft and under Diamter: 55cm for heights: 5'1 - 5'8 Diameter: 65cm for heights: 5'9 - 6'2 Diameter: 75cm for heights: 6'3 - 6'7 Diameter: 85cm for heights: 6'8 and above