Welcome to SMF. This is the place to get great advice on everything smoked. Please stop by the "Roll Call' area and introduce yourself. Let know a bit about yourself, what you like to cook etc.
As far as your question your minimum firebox volume would be 1/3 of the cook chamber (tank). 400 gal CC would be about 133 gal or a box about 31" x"31" x 32". This would be your minimum size, most people recommend going a bit larger (10 to 25% bigger) to accommodate for
ash pan, fire management.
There are on-line calculators to get your sizes , dimensions right. Try and search for Feldons BBQ pit calculator. There is a lot of help / opinions available here. Just ask.