That is a hard question to answer not knowing where you are going to put it and the dimensions of the room. You will need to know how much height you have in the area you plan on putting the bull in. If your ceilings are high, then height dimensions are not an issue. The size of the base will depend on if you like a large cabinet type base, or something that will just hold your mount up. The only real thing you must be concerned with is the balance and strength. You will need to be sure the base will not tip over, or not hold the weight of the mount. This isn't hard to do but clearly a concern..
When we built one for the wife's bull, our base was basically a 6-sided pad about 36"X36" that is only about 10 inches high, due to the height of the bull, and the height of our ceilings. We have a 4'X6' post coming up from near the rear of the base which give it good balance. The bulls nose is only halfway between the ceiling and floor, but with our dimensions, that is the only way we could do it and still have the mount in our house. A cathederal ceiling leaves the imagination to run wild with possiblities for different and interesting pedestals.
Good luck and keep us posted with a photo when you make a decision and get it done!