If the ball is too small, you risk stressing joints, including your knees. At this point your hips should be higher than your knees, which helps protect your lower spine from strain because you can maintain the normal curve of your lower back.
When you sit on top of the ball, with about two-thirds of the ball behind you, you should be able to plant your feet flat on the floor, shoulder-width apart. When you sit, it should have just enough “give” so that it sinks slightly and you can maintain your balance. Place a chair on the other side for stability, especially if you’ve never used this tool before. Here’s another way to check the size: Place the inflated ball in front of a mirror and close to a wall on one side.