ok. well. thats not what the margin does. you want to change the fullscreen mode.
the margin is jut your workspace in construct2. one of the full screen modes places the game top-left.
Actually the reason I want to increase the space margin is -- the game I'm planning to make may need a bigger layout with a small window 640 x 480, prearranged sprite objects will be coming from the outside and entering the window, it is a non scrolling type game. I was thinking if I remove the excess layout white space outside the window and since it is non scrolling, I might just make it plain margin space to lessen the file size and processing power of the CPU.