When seated on a fully but not over-inflated ball with your knees over your ankles your hips and knees should be flexed approximately 90 degrees, positioning your thighs approximately parallel to the floor.  Your hips should be level with or slightly higher than your knees. Below is a sizing chart that accompanies the exercise balls we currently carry.
 

*Standards from manufacturer Sissel Swiss Balls 

 Given the above recommendations are the manufacturer's standards, it has been our experience that most people we work with in our training practice that are between 5'0" and 5'10" use a 55cm ball. Many factors can come into play when selecting a ball besides your height. It may simply be a color preference. Or if you have weights available to you and are using them in conjunction with the exercise ball, you may need to consider the  weight recommendations for a ball. (Remember the total weight recommended for a given ball must be great enough to include both your body weight as well as the additional weight of the equipment you are using.) We NEVER recommend overinflating a ball even slightly passed the manufacturer's standards. However, with the quality of materials used in the balls we offer, underinflating them to  a point is fine. So if you think you might need a 65cm ball, but find your knees and hips are not positioned ideally with the ball maximally inflated to exactly 65cm, try deflating the ball to achieve ideal alignment. Do NOT overinflate a 55cm ball to achieve proper alignment however.