Diameter

I have a bunch of free duct that is a foot square.  Could I use the duct on the section that I am building the mass storage?  I would like to incorporate it in an outdoor bar.  Would it matter if it is larger than the burn chamber?

Greetings Paul and welcome to Permies!

Good question.
Anything downstream of the burn chamber will benefit from having a larger diameter exhaust path due to less friction of the exhaust gasses passing along the walls.
This has been the reason why many people have switched over to a bell or stratification chamber design where the gases are allowed to separate out on their own thereby almost entirely eliminating all friction and often lower exhaust temperatures too.
Let us know about your build as more information is known. We love to see new projects in the works and help out where we can.