Size of the mirror

In order for Mr. Hubert to see his reflection in the mirror, the rays of light coming from his head must hit the mirror, be reflected at that point ( incident angle = emergent angle) and then reach his eyes.

This condition is only fulfilled by rays that hit the mirror halfway between his shoes and his eyes. We mark this point with a 'B'.

The part of the mirror below this point 'B' is unnecessary because in this part Mr. Huber can see things that are in front of him, such as the floor (see picture below).