I have to agree with the others here. There isn't a formula for this. The best you can do is go by the manufacturers recommendations and read a thread or two about what others have used.
Example: I have a fairly heavy .40 size Cub. It's heavy because I painted it and did some scale detailing. The Evolution .46 that it came with flew it fairly well. I then went to a Magnum .52 that just wasn't powerful enough. I stayed at full throttle all the time with that engine.Then I put in a Saito .72, which has less horsepower than the Evolution, but the plane flies better with it due to the bigger prop. So there's a lot more to it than just getting the right amount of power.