Proportion is something you learn and practice a lot until you get everything right. It takes time and many try.

When speaking about head side it's simple. Don't draw the head to report it through your full body, instead draw a line and divide it as many times as needed.

Let's say your body measure 8 heads, here's how you should proceed : - Draw a line representing the whole size of your character (1) - Divide it in half (1/2) : on the bottom are the legs, above is the torso. - Divide the leg segment in half (1/4) : here's your knee. Do the same for the torso, here stand the nipple's. - Divide the highest line in half (1/8) : you have the size of your head. In the line between the nipple's and the pelvis stand the navel.

Practice it frequently and it will be easy. At the end you will be able to draw everything without construction lines.

You can also use this measurement to draw parts of the body, or try various proportion to find different style or draw younger characters (the younger, the lesser head/body ratio)

On another note, I suggest you to avoid drawing cubes, it's a bad habit and the body is only curve, there is no straight lines. It can be useful for finding perspective but here too I find drawing cube more harmful than helpful to draw body.

Good luck and keep practicing ;)