To answer the original question -- you chose the prop, based upon the engine/plane combination, plus the intended use. The engine should be kept in its usefull power band & the airframe, in addition to prop pitch & diameter, has a significant effect on engine performance. If you have a small, light, clean airframe you can overrev in-flight, despite an acceptable static RPM -- in that case you could use a smaller diameter "pitchier" prop than you would if the plane was blunt & draggy. In the latter case, an increased diameter & lower pitch prop may enhance airframe & engine performance. If you need vertical, increase the diameter & flatten the pitch. If you want speed, decrease the diameter & increase the pitch (assuming that the plane is clean enough to actually go significantly faster). If you need take-off performance (floats, deep grass, etc.), go with the longer flater prop. If you have long hard runways, you can possibly use a smaller steeper prop, if the ac will support its in-flight characteristics. Altitude & temperature also play a part in prop selection. As engine performance falls with increasing altitude & increasing temp, a flatter prop will probably be needed - with or without increasing diameter, depending on case specifics.

In your case, an 11-6 is probably a good starting compromise for your engine/plane/usage combination. However, there is a substantial degree of trial & error involved & a typical BB ABC .46, like your EVO, can use a wide range of propellers, from a 9-8 to a 12-4, depending upon airframe & in-flight usage. Increasing diameter raises power load faster than increasing pitch. As far as the engine is concerned, too little load will send revs soaring past the usefull range, & possibly into the destructive range and too much load increases piston side-load, increases crankshaft radial thrust loads, & increases cylinder temps & pressures -- all of it bad.


What could be simpler?