RE: .15 size pylon planes
don't take this as gospel, but we used to take Rev-up 7-5N (narrow) and they'd end up about 6 1/2" diameter or so - or you could go with a little more blade area, take the top flight super maple 7-4 1/2, trim them down, narrow the blade a little bit, thin 'em out, etc.
Towards the latter part of the racing, the props were molded continuous strand carbon fiber, about the same size, but they could be a little thinner because they were stiffer and stronger.
Somewhere I still have a prop or two.. not many, but I think I do.