The size of an exercise ball you require is determined by the activities and exercise you want performed.
A properly fitted ball for seated activities that require the feet on the ground should seat an individual with his/her hips, knees and ankles at 90 degrees. A simple solution is that the distance between armpit to middle finger tip is the ball diameter required for that person.
Balance activities with children may need a larger size ball so they can balance on top of the ball.
Colours will vary.