Diana, if you mean a stability or balance ball, you want a ball that you can sit on with your thighs parallel to the floor & your shins perpindicular to the floor. Your legs should form a right angle. At 5'4", I actually would do best with a 60cm ball instead of the 55 or 65's that are available. The boxes give guidance on what size of ball you probably need based upon your height.
Debra