Just go with something the size of his widest point, odds are if it can handle 2 pinkies, it can handle a fuzzy. 1fuz = 2pink... 2fuz=1hop etc "in general" (of course, not all mice are created equally )

As far as cage aggression and tail rattling... sounds like this evil baby albino cal king we have. He would mock strike and musk and squirm but every time I came around to his box I would just slowly pick him and hold him for a minute, about 3 times a week. 2-3 weeks later he's a complete doll. I think it's important not to overdo the handling when trying to get a snake to settle down, as to reduce stress. They call me the python whisperer at work sometimes cause I'm so calm with the some of the snakes lol Then again they call me crazy a lot for it too loool