You'll know what the proper level inflation for your ball is based on its circumference. The stability ball will always have it's proper size printer on the side of the ball; and it's usually listed in centimeters. It will have 45 cm, 55 cm, or 65 cm on the side. You'll inflate the ball, then put a cloth/lightweight measuring tape around the middle exterior of the ball to see what it measures to. If the circumference is less than the recommeded size, inflate it more. If it's a lot more than the recommended size, let some air out to avoid bursting it.
I'm a certified personal trainer who inflates a lot of these at work. Hope the info help.
The 55 refers to the diameter of the properly inflated ball in centimeter.
ie the distance from one side of the ball to the other side.
Take a yardstick and meaisure 22 inches from a wall in your house.
Place a piece of tape on the floor at the 22 inch distance. Now hold the yardstick verticle straight up in the air with the bottom on the piece of tape. The ball is properly inflated when the ball just fits between the wall and the yardstick. The 22" is arrived at by dividing 55 cm by 2.54 which will give you the inch equivalent of 55 cm.