Chasing BBQ,
There is a rack a few inches up from the floor where the charcoal goes. Underneath the charcoal grate, I have 4 1" pipes with valves to control the heat. I added another rack above the charcoal to put a water pan to help disperse the heat better. There are two cooking grates, the first one is about 27" from the charcoal grate and the second one is 8" above the first cooking rack. Other than the waterpan, the rest is wide open. The opening inside the smoker is 16" wide by 24" deep. It maintains temp pretty well, I'm still trying to perfect it. I start it off with one and a half chimney's of charcoal. It takes about a half an hour to an hour to warm up. When the temp dips towards 200F, I shovel in some lump charcoal and open the intakes a little. hope this helps. Also, I got a lot of the details of my build from the Wilbur Hog smoker (including the door design). Here's the link which includes drawings.
http://www.ibiblio.org/lineback/bbq/wdh.htm