I need to get new grips for my kid's bike, but how do I know what size?

[pinkoos]

How does one measure the size? And, what size are they quoting when they something like "137 mm?"

This is just an example:

http://www.amazon.com/Avenir-Comfy-Grips...ting-goods&ie=UTF8&qid=1315926780&sr=1-2