The burn area on a gravity smoker is really small. At the bottom of the chute is a grate that keeps the burning coal from falling into the ash pan until it is pretty much burned to ash. There is still enough hot coal falling through to ignite wood chuncks though. Right above the grate is a tube that takes the heat from the burn area to the cooking chamber. There's an airtight lid on the top of the chute to keep the chute full of charcoal from burning up. On my cooker the burning area is 6" x 6" x 4" tall.
Lang 84D, Homebuilt Gravity Fed, WSM-18, Weber OTS