The proportions of the human body vary from one individual to the next depending on body size and age. But there are some rules of thumb that will help you in your drawings.
You can figure that the height of a grown man is equal to about 7 or 8 times the height of his head (the man shown here is a tall man). A 10- or 12-year-old child’s height is only about six or seven times the height of their head. When the arm is outstretched, the tip of the fingers comes almost to the mid-thigh.