I agree with Norco but it really comes down to how much is open when you're smoking. I have one 1 1/2" hole and I have it wide open to start and then close it down to 1/8 to hold 225. I would think 2.25 might come up to temp faster but it depends on the exhaust also. What are you using to control the air? I think I would use sheet magnet or a sheet metal flap rather than a huge ball valve. I kind of like a nice clean look to the barrels without too much hanging off them yet my favorite barrel to cook on has a intake coming up the side.
Let us know how it works for you!
How high- 2" off the bottom seems to work good to keep ashes and juices in the barrel.