If you do a square hole, the radius at the corner should be max 0.012" according to
spec, which would need a 0.6 mm bit.
But there are alternative shapes. Some shapes have been developed primarily for giving the housing's locking tabs some play so that the switch could be opened for modifications without first having to remove the switch from the keyboard.
For CNC, if you don't have a 0.6 mm bit I would propose a laying down H-shape. Read more
here for various test that have been made of that shape.
Edit: Then check
here for another discussion on shapes for CNC.
