How to figure out my lense size?

I would like to get a fisheye lense for my nikon D7000. I don't know what mm to get I have two lenses thus far 18-105mm and a 55-200mm. Whitch lense would be better to use the fisheye lense with? I think the 18-105mm would be but i'm still learning the lenses.

Neither really.

A fisheye lens will cost you around $800 and up if you get a true fisheye, one that can "see" 180 degrees

You are asking for a cheap (under $40) add-on lens which may or may not see a full 180 degrees.

If you buy a 0.25x lens and do the math, the 18-55 mm will give you the best chance of getting what you want. The filter size of the 18-55 mm lens is 52 mm

Don't put a junk filter on your good camera. Get the Nikon 10.5mm DX fisheye. I have that lens with my D90, and it is an excellent lens.

http://www.althephoto.com/lenses/fisheye.php

The D7000 would work well with the 8mm f3.5 fisheye that's sold under various names (bower, samyang, rokinon, etc).

A screw-on adapter would probably do something fishy on your 18-105 if you don't mind loss of most of your image quality. It takes 67mm