Around here, we use the line sizes per The AMA Rule Book:
Thus .015" for .15's and .018" for the bigger engines.
However, this is something of a moot point in that the 15's don't seem to be at all competative. Sure they fly OK alone and without a streamer. But with equal-size streamers, the little engines take a big hit on streamer drag. Not to mention getting the streamer in the prop if they do score a cut, or worse yet, snagging a streamer with the lines. Did we talk about what happens in line tangles?
The guys who run FP25's and choked-down 36's welcome anybody who wants to take a poke at 15's in Speed Limit.