Firebox question

[crestliner]

Ok guys i found my tank, it is an anhydrous tank 4 feet by 7 feet. The question i have is why does the firebox have to be close to 1/3 the size. I have seen some smokes that have only maybe 7 or 8 splits to hold the right temp. Is it for the correct draw or other reasons? Also if i did the right calculations i think i need around a box 34x34x34. Thanks for any input.

 

[duck killer 1]

i think it's for the correct draw but also if the smoke stays in the cook chamber too long it could cause soot like build up on the food and give a bad taste / odor. maybe some of the pro's will have some more insite.