It's your pit, but I believe you'll find if you use 2 2 inch pipes on a cooker that large, you'll wish you had gone larger.
My unit has a similarly sized firebox ( 20 inch pipe, 24 inch long) and roughly the same size cooking area (main box is 32 inch pipe 4 feet long and and attachad rib box 20x20x36 or so) and I use 2 5 inch pipes for exaust.
As Dutch said, you want unrestricted exaust. Restricted exaust = stale choked smoke and bitter flavor. You want easy fast flowing smoke..especially in a wood burner.
My best guess on the volume of the exaust being what matters.. would be draw. In general, the longer the pipe, the better the draw and the faster the smoke can move thru it. And I buy that to some extent...but in the above example, his firebox has a volume of 11304 cubic inches and needs an exaust volume of about 565 cubic inches according to the spreadsheet. I have a hard time believing that a 720 inch long 1 inch diameter exaust pipe will vent as well as a 20 inch long 6 inch diameter exaust. I know which one I'm using..if I'm building a new pit.
btw thanks for the link..tells me that my inlet should be about an inch bigger than I currently have it...