I look at what the scale prop diameter is for the plane I am going to build, and try to use an engine that will turn that prop without overloading it. take into consideration the type flying you will do. If it's a high performance plane and you will be turning at high throttle settings, go with more engine than you would for, say, a Cub, where you would rarely be running the engine wide open; but also consider where the engine's power band is. If it's a 2 -stroke, you'll have to turn higher rpms to make the power required to turn a bigger prop, if it's a 4-stroke, you can get by with a more marginal engine because the powerband is at lower rpms (if you don't need the speed, as in a Cub, etc). If it's a gasser, then you have even more latitude because of the torque curve.