The fingers reach a little below the center of the thigh. In children the head is larger in proportion to the body. The central figure in the diagram above, the one year old, has a head one-quarter the length of its body. You can look at theses diagrams and study your own body, but there's no substitute for drawing from a live model. Portrait painting is really only an offshoot of life drawing - a sort of life drawing with personality superimposed.
Some students are interested in anatomy, the study of the skeleton and the muscles.
Others don't have the slightest desire to know what goes on under the skin. If you want to study it in more detail, I have studied anatomy for 10 years and offer a comprehensive course. But you don't need to master it in this great a depth. Just have a general grasp of the details. Practice drawing people of both sexes, all ages, seated, standing, in a variety of positions.