How can I tell the size of a bike's axle?

[Peanut to the rescue!]

I've got a rather small bike that I want to put pegs on. The pegs only fit 3/8" axle and I have no idea what in the world that means. Is 3.8" the length of the wheel's axle?
Also, it's not a bmx, and if anyone gives me a link from lmgtfy they're dead meat. =P

 

[Bob A]

The 3/8" is the axle thickness, use a 3/8 inch wrench to see if it's the same size.