Jim,
I'd recommend a range from .008" absolute biggest (I rarely use it anymore and prefer .005" as the "top" end) down to some of the lighter stuff down at .004" (2.4 lb test Dai-Riki. You can get the Dai-Riki Products at Bass Pro Shop or Cabellas in the Fly Fishing Supplies; I believe their main use is in fly Tying. Remember, the heavier you choose the more tension is required to get the line tight, and often the components won't take the tension. Also, if you are thinking of using Brass tubing for turn buckles .005" is about the max you can go with .5mm tube.
Others will have their own preferences however if I had to use just one it would be .005" / 0.13mm which is 2 pound test normally.
Hope this is of some help.
Cheers,
Lance