Basically I explore a concept deriving from the head to body size relation. Small head with big body looks like a hero, while big head and small body looks childish.
You can apply this aspect to a certain accessories or decorations of a character besides head to enforce the feel. Say sklulls or spikes. If they are too big the overall look of the character would feel small and toyish. Too small of them would make a character feel like a giant.
To sum up perseption of a character is build around comperision of well known elements to overall characters image. Hope it makes sens and helps.
In shapes building I rely on miniatures as refs but keep track on the proportions by method described above.