I agree with you it's just a matter of trial and error, example when a plane recommends a setup such as 3s and 12x8 prop you know you have a margin to play with like maybe up to a 13 inch prop or even down to a 11 inch prop what comes to be a major factor is the pitch, I was always told if the recommended prop pitch is a "6" for example if you increase diameter you need to decrease pitch and if you decrease pitch you need to increase diameter, I don't know how well this really works out