Are you trying to draw from a reference, or from your imagination? Many people have this idea that "real artists" can make things up out of the clear blue sky, but honestly, that's rarely the case (and even when it is, the things they've made up generally aren't realism.) If you want to get the proportions right, look at a real person -- in life, or in a photo -- and
draw what you see.That said, of course there are mathematical relationships that can guide you.
Here is one of Brenda's lessons that spells out such a system.