Gas engines make less horsepower per displacement and weight than glow engines do, so you do have to go bigger when changing over to gas from glow. Choose the engine that has the power and weight you need for the plane, then pick your prop. Just understand that if you go gas you will be adding some weight onto the nose for the same horsepower which also means adding some weight to the tail to balance it. If the plane you are working on is marginal on wing loading, you may not like the result.