Exercise balls cost around $20 and are widely available in health
and fitness shops. Be warned: all balls are not created equal.
Some are cheap to the touch and to the smell. The appropriate size
of the ball is a subject of debate among teachers and depends on
what exercise method you will be using.

Balls are blown up according to diameter, not air pressure. A yardstick will help you inflate to maximum diameter (height off the floor), which is printed on the ball and on the box. Only inflate t the recommended diameter and not bigger. Use an air raft or mattress pump or go to a gas station and use a cone-shaped trigger nozzle. Most people find that a bicycle pump is simply not forceful enough.

Here is a rule of thumb for choosing a ball that is the right size for you:

With Pilates-based work the 55-cm ball is perfect unless you are very tall (and strong). The larger the ball, the heavier and more unwieldy it is.

A general rule is that when sitting on the ball the hips and knees should be bent at a 90-degree angle. This usually translates to 55 cm for people 5′ to 5′ 8″ and 65 cm for 5′8″ to 6′2″.

The author of this article prefers the Fitball brand. She loves the pearl- or black-colored balls because they have an excellent surface that is not dangerously slippery.