since most of your wood will be cut at 24in. it seems to me having a box atleast that long could save a lot of trouble In cutting wood to fit. i suppose you have to weigh that vs efficiency loss if a smaller fire box is recommended per the calculator.
I am planning my first build with a horizontal 120 gallon tank and a vertical warmer/smoker over the firebox. I plan to damper both the horizontal and the vertical. My thoughts are that I can smoke in the horizontal unit while using the vertical as a warmer, or smoke in both the horizontal and the vertical units at the same time, or smoke in only the vertical unit all by opening and closing the dampers. Has anyone tried this? Do I need to calculate the firebox large enough for both the horizontal and the vertical units? If the firebox is large enough to run both units, will it be too large to only cook in one smoker? I would appreciate any comments or suggestions.