Using the right size ball is important, some people seem to think bigger is better but actually the correct size of ball is based on your height because when lying down this effects the position of your back and pelvis

If you are less than 175cm tall (5 feet 8 inches) then go for a 55cm ball. 176 – 195cm (5 feet 8 inches to 6 feet 3 inches) then you need a 65cm and if you are 196cm (6 feet 4 inches) or more then you need the 75cm.

If you are going to use the ball for sitting at a desk (which is a fantastic idea) then you can go for a ball one size smaller so that you don't have pressure on the back of your legs when sitting on the ball.