Warmouth, I think it's up to you and the action you want from the legs. I try to leave them a little long on most and cut them streamside if they are'nt working or getting wrapped around the hook.
As for size! I'll say that small or ex'small legs are not as strong as larger sizes. I tied up some turk's tarantula's in size 12 with small rubber legs and size 10 with medium. The bluegill really liked the size 12 but quickly made off with some of the front legs the other day. This did nothing to stop them from hitting the fly. The medium legs seem to last longer on most of my flies. But you can't put medium legs on a #16 size fly. I hope this helps some. chad