sqiddster
I might have misunderstood the question but doesn't the "set canvas size" action already do what you want?
As in
Event-->On start of Layout
Action-->Set canvas size to WindowWidth x WindowHeight
I believe this takes your current window size and sets the canvas size to the window size.
You could take the WindowWidth and WindowHeight and adjust it by subtracting by some value or divide by some value to scale down its resolution.
You could maybe also switch out the value of WindowWidth with viewportRight("layer"), and WindowHeight with viewportBottom("layer") to get the devices viewport on the browser then adjust the canvas to the viewport.
Do you think that might be what your looking for?