36"
. It will need a roof brace too.
I was curious about the space. Is it always heated so that you are starting the fire when it is say 60F? Or is this space unheated and the fire being started at a cold temp, like freezing or below?
For some guidance on how to start and run the stove, here's a nice video. Choose the bottom one titled: Efficient Wood Stove Operation.
http://www.woodheat.org/wood-heat-videos.html
You'll note that once the fire is going well, the air is not reduced all the way, but just to the point where the flames get lazy and kind of waft over the wood. That will encourage stronger secondary combustion. At that point the fire will start to pick up speed again and may need the air reduced, slightly once again. But not to the point of snuffing out the fire. You should see the flue temp decline and the stove top temp increase. With good wood the stove top should run about 550-650F and the flue temp about 3-400F once you are in a steady secondary burn.