We have a 30 x 48 pipe (rounded ends). Was figuring a 20 x 20 (id) firebox would work. Was trying to figure out how big to make the opening from the firebox to the chamber.
I do know that you need the same square inches going in as you have coming out. There's a ratio chart on the web somewhere for it. I will try to find it or maybe someone will chime in who knows all that info.
Maybe this will help or maybe there is a better one chart. Just google ratio chart for building a smoker. I know this is in a spreadsheet form and you can enter your dimensions and it will calculate for you.