I'll take a stab at this, too.

Your engine is going to have a certain rpm where it performs best. The idea is to choose the diameter/pitch that will put you at that rpm during your preferred flight profile. So there are 3 pieces to the puzzle that you are trying to optimize: engine, prop and airframe.

Your propellor's diameter is usually determined by ground clearance. You need to be able to take-off without digging holes in the runway. So for a .40-.50 sized plane you're diameters will be 10-12". The pitch is determined by engine loading and airspeed. The faster the plane moves the more the relative angle of attack of the prop blade changes. The net effect is that you need to add pitch as speed increases, but this increases engine loading so you should also reduce diameter. The pitch ends up being limited by flow separation. If you spin a blade with too much pitch too fast it will stall.

As a rule of thumb for .46 planes, a trainer would take a 11X5 or even 11X4. If you have a strong .46 you could probable turn a 12" prop, but I have found that my OS .46FX likes 11" props better, a sport plane would take a 11X6 or an 11X7 if it's real fast, and a 3D plane would take a 12X3.5. So you see, the faster planes take more pitch while the slow-flying 3D plane takes a real flat prop.