Coin size ( length )

A coin is round and doesn't have a length. I'm assuming you mean the diameter.

What do you mean by "how" do you measure it?

Simple, you measure its diameter then proceed to use the following:

2 * pi * r where r is half the diameter. This gives you the perimeter of the coin which I suppose is what you meant by length.

And if you want a precise measurement, you can make use of a pair of vernier callipers.