I run a 68" 3 blade on my Rotax 503, I'd estimate around 300lbs of static thrust at sea level..

I had an OS 1.40 RX on a fun fly plane a while back which I ran an 18x8 wood prop and it gave pretty much unlimited vertical performance, that was a pretty wowie performing motor (fell victim to bearing rust tho).

One day if I get another plane and radio, I still have a NIB 1.60FX that I'd like to run, I think it'll swing an 18" pretty well....

LS