Wayne, size "A" is standard and good for just about anything and it is usually nylon. Some people, bamboo builders especially, use 3/0 silk, which is 1/2 the diameter of size "A" nylon. And some of the finest builders I know even use 6/0 silk, which is even 1/2 of 3/0. Keep in mind that if a thread is thinner or smaller, more wraps will be needed and that can actually add to the strength as long as it's not too thin. Lastly, you want a round thread, not one with a flat profile.

In other words, it's ok to experiment and the finish, usually a two part epoxy, will be absorbed into most threads and make a really strong wrap.

If you're planning on tuna rods, it's a different story . . . .

Ron