This is just a rough guess, But i would have to say after looking at the cranial cavity of a deer's skull it is probably right around the size of half of your fist. Mind you this is just a guess, but it'll get you in the ballpark. I'm sure somebody here can give you a much more specific size.
Ohh, and I just noticed your new here, like me, so welcome tothe boards...lots of nice guys here.