Well, it depends a lot on what style of artwork you're doing. As an example, I read a cartooning book awhile back that had the height of the head equal to the width (and sometimes greater width than height), larger eyes that were a little lower on the face than normal, and the ratio between the heights of the head and body as around 1:2 or 1:3 (don't remember which... it's been awhile).
But I'm sure it'd probably be much different if you were working on a realistic image, or manga style, or something like that. So what type of artwork are you attempting?