If object > Is on-Screen(inverted) > do the thing
As for memory usage:
Off-screen objects are not still rendered. Construct 2 does not issue draw calls for objects that do not appear in the window, and the GPU is also smart enough to know not to render any content that appears outside the window - even when a single image is only partially on-screen.
The layout size also does not have any direct effect; larger layouts do not use more memory or require more processing, unless you use more objects.