I plan to build a reverse flow smoker from what appears to be a 200 gallon propane tank. The tank is 30" dia. X 87" long. I did the calculations on Feldons using both 200 gallon tank size and 30 X 87 cylinder. There appears to be quite a bit of difference in cooking chamber sizes and recommended fire box sizes. I put 24H X 30W X 30L for my fire box calculations and came up with 140.3% size differential for the 200g and 105.4% for the 30 X 87. I actually intend to build the fire box 30 X 30 X 30 but put 24H in my calculations to allow for space under the grate for air flow.
From what I've read here so far a fire box that is oversize is ok, it just doesn't need to be to small. I plan for the fire box to be my 1st step of this build.
I'm going to have to figure out this cooking chamber size a little closer before I start cutting openings and putting baffles in the tank.