going by size alone for engine size... bad idea. If you went by size alone for putting an engine n a Sig Kadet Sr... you might stick a .91 into a plane intended for a .40.

compare the aircraft weight with the weight of similar design aircraft of similar size to find one that has an appropriate engine size. (allowing 12 oz roughly for engine if none is present... or stick in lead where the engine belongs till it ballances, and you'll have the appx engine weight to help in the guess.)

You can guess about engine size a lot... but that model seems to have an engine in it already... what size does it have? What prop is that on it?