HAMED,

Those big engines are used on large aerobatic airplanes mainly. These planes need a lot of power to perform all those difficult maneuvers, although they are not all that heavy.

An a not to heavy built 2x2 meter sport plane you can get by with something like a 15cc two stroke engine.

Why don't you look up some manufactures of airplane kits on the internet, and see how the power range of engines correlates to the size and function of the model. That should give you an idea.