I've been looking for a tennis racket recently and after some digging on the internet discovered that you need to know your grip size to get the right racket. Basically when you hold the racket there should be a finger width's space between the tops of your fingers and the rest of your hand - you can find illustrations of this on the web. So grip the racket and if your finger tips go all the way round and touch the other part of your hand then the grip is too small. But if the distance is more than a finger width it's too big.
Grip sizes go from 4" - 4 3/4" (going up in 1/8 inches), so it's quite precise. However, I found that when I was looking to buy, it was only the more specialist that had these measurements on them, a lot only had small, medium or large, so you kind of had to measure it yourself.
Hope that's helpful.