Agree with ChuckW. If you already owned some glow engines, you can put the same prop on them and measure the rpm with a tachometer. This is a little bit useful for comparing the power of one engine to another engine. But if you just buy the largest displacement recommended by the airplane designer, you should be OK. For example, if the plane says ".40 - .60 two-cycle glow recommended", buy a glow engine about .60 cubic inches and you will be fine. You can even go higher than the recommended if you want but you will also be adding weight.