Yes, I was teasing about the hands…I knew why.
Tails are harder than they look.
As for head height, consider Darkness in Legend. Darkness is one of the best looking demons in movie history, and his head is huge. The reason demons like darkness and Tom have such big heads is because they’ve got huge horns, which require a very large/stable base. So unless the brain is bird sized, the head has to be really big to fit both brain and hornbase/bone structure.
Assuming, of course the rules of biology apply to Abyssal demons, because I note that Tom’s brain seems to work, even when he’s insubstantial fire, so maybe he doesn’t require an organic brain in his body.
Blender is great, it’s one of the better (if not best) free tools for modeling. You could also check out DAZ Studio Pro at http://www.daz3d.com it’s not a great modeller, but it does do rigging, morphs and rendering….of course it’s free so they can tempt you into buying their content….