Mark, It varies depending on the size of the hog. You have to plan on 6 to 8 inches for the coals....then measure enough to be able to lay the wrapped hog or meat in it and still get a sheet of tin over the top without touching the meat.
It's kind of a start digging, measure the pig and go from there deal. It's better to have it too deep than not deep enough.
Hope I didn't make this as clear as mud!