Quick Answers:

When you look in a mirror, you see twice as much of yourself as the length of the mirror.

Also, when you look in the mirror and then back up, you see exactly the same amount of yourself.

Complete Solution:
The drawing shows that you see twice as much of yourself as the length of the mirror. Suppose that the mirror is hung flat against the wall so that the top of the mirror is halfway between your eye and the top of your hat. Lines 1 and 2 show the "line of sight" from your eye to the mirror and from the mirror to the top of your hat. Lines 3 and 4 show the "line of sight" from your eye to the mirror and from the mirror to your foot.

The horizontal lines show the heights of the mirror's top and bottom. The distance from the top of your hat to the horizontal line indicating the top of the mirror is the same as the distance from this line to your eye. The distance from your eye to the horizontal line indicating the bottom of the mirror is the same as that from this line to your foot. Thus, you can see twice the length of the mirror.

Try drawing the figure closer to the mirror or farther away. Although the angles change, the part of the body seen in the mirror is always twice the length of the mirror.