The goal in pit building is make a cooker that works for you and not works you. Understanding the dynamics of cooking and heat and airflow. Understanding combustion. I can take a 20x20x24 firebox attached to a 48x23 tank, fill the firebox packed solid with wood and get cook times like a pellet cooker. Because it's not an issue of how big the firebox is. Its giving it only enough oxygen to burn just enough wood to give u the heat u desire. That can be accomplished with dampers and or an exhaust damper. U can nearly close the firebox dampers yet open the exhaust and suck air into the firebox. U can also choke the exhaust open the dampers and run the firebox hotter. My point is, no calculator, thread, fellow cooker or pit builder can give you a cookie cutter formula for size. It's about understanding smoking and designing a pit that meets your cooking style. But no matter what, u have to understand the dynamics and principles relating to air flow and combustion before u can be a successful pit builder. Look at it as an engine or carburetor. If u take a 350 V8 engine and put it in a VW Beetle, it will be fast as hell but not efficient because the design of the larger engine. If u put a 2.0 4 cylinder in a Ford F350 u will over work ur engine trying to go anywhere because its underpowered. So determine what u want the cooker to do first, then design it with that goal in mind