What ball size should I start with?
The Miracle Ball Method book has the 4 inch balls included. It is a good ball to start with to learn the method. The amount of air in the ball is personal preference. It does not determine the success you will have with the method. No two bodies are completely alike. Some people prefer a firmer ball, and others find that the ball must be almost deflated in order to use it comfortably.
The valve on the ball is to adjust the amount of air that works for you. Whether firm or soft, you must allow the part of your body on the ball, to feel its weight supported by the ball. Then follow the directions provided. The balls can be adjusted with a standard bicycle pump.