I have an OS70-fsII and I use 13x6 props on it, I would think this is the standard mot people uses for this engine ?!?
Right now I'm too building the SSE kit and have ordered another OS70 engine for it, I plan to use the stock landing gear and a three bladed 12x6 prop.
I have used 3-bladed 12x6 prop on my other OS70 and it's a good match.... I'm not into 3D and need power to hover and I think the .70 will do just great even with a less effective, 3-bladed prop.
Advantage is that I can use the stock gear + it looks cool with three blades