its about 2 3/16" outside diameter. It doesn't come with a wiring harness. If i recall correctly when you buy a new one it doesn't come with a harness either. I also think that is true, because of the "wiring diagram" on the back. The backside of the guage is just a standard male wire terminal. So all you need is a length of wire, and three female crimp style terminals...pretty straight forward.
Thanks,
NAte