How to animate the size (width and height) of a rectangle animation in after effects?
Why not just start with a full screen square and simply position it off the bottom of the frame.  You can then simply animate it moving up.  Even if you have something on the square, it will generally look better to slide in than stretch in.  If there isn't anything on the square, it will look the same either sliding or stretching in if I understand your scenario correctly.