If would like a fx:

Keyboard (E) ---- And ---- Game (Set window size)

So when you click a button it change the windows size.

In the game actuator you can under the "Game:" use "Set windows size" and then i change to x and y axis, where you can type the size.
I know that you can use python, but it would be easier with that.
I hope you understand what i mean :)

-=TheZCGamer=-