First thing you need to do is draw your model life sized. Whatever it may be. In this case it is going to be a scale model of a human.
Coming from a fine art background, I know that the "ideal" human figure is 7.5 heads tall. Meaning that if our model is going to have a head 1in. tall, the overall figure is going to be 7.5in. tall.
You can search online for human proportions and find reference photos to help you out. I used this one
to help guide me.