The DLE 20RA is rated 2.5 hp at 9000 rpm. So allowing that the engine will unload in flight and gain 300 to 500 rpm, I would be looking for 8600 to 8700 rpm static for peak horsepower. However, a larger diameter lesser pitch prop running at lesser RPM may often out pull a prop that turns at peak HP. The only way to tell is to buy several props and experiment. Of course the props listed here are 2 blade props. If you are looking for a 4-blade prop, you would need to consult the prop manufacturer's tables. But either way, it will not perform as well as a 2-blade prop.

Engine DLE 20RA
Displacement 20.5cc
Power/RPM 2.5 @ 9000
Thrust-LB 13.9
Weight 1.92 lb
Fuel/Oil 30:1
Prop 14x10, 15x8. 16x6, 17x6, 18x6
Idle/Max RPM 1700/9000
Plug CM6