Engine sizes are the number usually found somewhere on the side of the case. A .40 is 4 tenths of a cubic inch displacement, .25 is 1/4 cubic inch displacement. Obviously the larger number means a larger engine and the smaller # is a smaller engine. In the early days the displacement decided the class in which the engine could be flown. Class A up to .20 class B up to .30, class C everything else. These classifications were modified and now are used only in certain aspects of contest flying such as free flight and control line or speed.