First and foremost the model has to fly stalling or snap rolling within an acceptable range of speeds and turn decently tight without G force accelerating the loads to a high enough degree that the model stalls and snap rolls out of what would be considered as "normal" turns. From there you pick an engine that is capable of lifting that much weight in an acceptable climb rate. It's probably safe to say that you can get away with a lower power loading on larger and lighter models like old timers while smaller models with a higher wing loading will require a higher power loading. But at some point the model weight and power loading will result in a model that may fly but it sure won't be any fun to fly.

As mentioned already you can't go wrong by looking at similar designs, styles and sizes of models out there already and just use the same engines those folks are using.