I have a package that I got from a purchase of someone else's fly tying material that they were selling and the package I have shows that there are 4 sizes: Medium #2, Small #1, Mini #0 and Micro #00.
I have the Mini #0 and they are very small. To give you some idea of the diameter of the Mini #0, I put it up beside some 12 pound test mono that was .013 in diameter and Mini #0 is almost the same diameter or maybe a little bigger, but, not by much.
Hope this helps...
Warren
Fly fishing and fly tying are two things that I do, and when I am doing them, they are the only 2 things I think about. They clear my mind.