Firebox

I asked Ritch Robin this question a few months back at a comp. The short answer is no. There is no rule to firebox size. Air intake and exhaust stack size yes. A 20"x20" firebox would be enough for all but the largest pits. Most pit makers make the firebox symmetrical to the rest of the pit so it looks balanced. The firebox on our 30"x8' comp pit with a 30"x30"x40" upright is 30"x30"x30" but we actually have a very small fire in there. A 20" would serve just as well.

Air intake on an offset like that could be two to three 4" holes with at least that much in exhaust. A larger exhaust is common. Most folks run the intake dampers and flu wide open and control the temp with the fire size. Good airflow helps develop blue smoke.

"Good airflow helps develop blue smoke."
Zilla

Now that piece of wisdom right there is worth it's weight in gold.:eusa_clap

I would take into consideration what fuel you will be using. If logs what size and build accordingly. This link might help decide or confuse.
http://www.millerwelds.com/education/projects/mboard/showthread.php?t=1835&highlight=firebox

B-I-L's pit has too small of a firebox in my opinion. Max temp on a good day is 220 on the cold side, that is with the firebox loaded and the dampers wide open.

I saw a pit running for four hours without being touched, had about 5 split pieces (12-18" long) in it to start, closed the dampers a little, stayed at 220 on cold side, 250 on hot side, nice blue smoke. I was impressed, but never got around to seeing how it was made inside. It looked like a pipe ran the length of the cooking chamber from the firebox.