Unless you mean how to change the frame size during runtime, but you didn't specify that so you are an idiot and I have no idea how to do that anyway kthxbai
If he's designing an application, he wants the level editor size to match the size of the window, in case you resize the window. That way if you resize the window, you don't get the black boarder, but you want to be able to define the sides of the frame as the sides of the window.
In MMF2, this can be changed by runtime, it's under the storyboard options: