I think what you need to know is how people measure their own grip sizes. Grip size refers to the thickness of the racket handle. Size 0 (4" circumference) or size 1 (also marked 4 1/8, meaning 4 1/8" circumferece) is the smallest, while size 5 (also marked 4 5/8, meaning 4 5/8" circumference) is the largest. Of course the larger your hand is, the larger grip size you need.
The best way to measure your grip size is, in my opinion, by feel. However, assuming you don't have any "feel" to speak of, there's one way to quite accurately measure grip sizes.
First you need a racket (or some rackets, with known grip sizes). Open you racket-holding hand. Put your hand on the racket face, making sure your palm is parallel to the racket face. Then slide your hand downwards (keeping this hand "angle" and grab the racket handle. Not too tight, but not too loose; just the kind of tightness you imagine you will hold/squeeze the handle when you actually hit a tennis ball with the racket. There's should still be a little gap between the four fingers and the bottom part of your palm. Now put the index finger of your other hand into this gap. If it fits perfectly, you've found your perfect grip size. If the gap is more than one index finger, the handle is too big for you; if less, the handle is too small.
If you are in between grip sizes (e.g. there's less than one index finger in the gap for a size 2, but more than one index finger for a size 3, meaning possibly a size 2.5), usually you should go for the smaller size (in this example, it's size 2), unless you have a much better "feel" when holding the size 3 racket handle. It's much easier to increase grip sizes than to decrease (which I don't know how). Using "heat shrink sleeves" can increase 1/2 or 1 size very easily (although adding some weight to the handle and hence affecting the specs of your racket).