Size of the mirror

Solution:

Mr. Huber only needs the part of the mirror between the points A and B where the rays of his head and his shoes hit the mirror on their way to his eyes.

This part of the mirror is exactly half the size of Mr. Huber. In the picture below you can see the mirror just as big as needed.

If the width of Mr. Huber is half as big as himself, Mr. Huber should hang the mirror upright. If the size of the mirror is not tall enough, Mr. Huber should find a different Mirror in his apartment that is at least half as tall as he is.