IMHO the easy way to set up the proportions is to put a 28mm human model side by side with your prototype, and make the later just one (human) head less tall. Then sculpt it as bulky as you want, but only one head lower. This is visually enough for a sense of high, and you will have still room for manouvering in areas often neglected, as legs, thights and knees.
Good luck with them, sir.