The law of reflection
Consider a light-ray incident on a plane mirror, the angle of reflection is equal to the angle of incidence. This is the law of reflection.
When looking at the end of the head and feet with a flat mirror, the size of the mirror needed is only about half of the height. So, you do not really need a full-length mirror as big as your height.
A normal line
Draw a hypothetical line perpendicular to the mirror surface and call it ‘a normal line’. The reason for introducing the concept of a normal line without using the surface of the mirror is because the surface of the mirror is two-dimensional, and the normals can be clearly defined even when the mirror is bent.