Insulated vertical offset smoker.

I used a 4"x8" opening in the masonry wall below and between the smoke chamber and the firebox. I to, had difficulty determining this opening size for my build that is a little larger than what you are building.  I looked a various plans and various commercial units to see what was used.  This was my best guess and it seems to work. 

Cubic inches is the measure of volume and does not relate directly to an opening size.

The baffle is the wall between the firebox and smoke box that diverts the air/smoke flow down and then up into the smoke box.  The flow is created by draft that occurs in the smoke box from the chimney above. In most smokers I have looked at the chimney is relatively small, like 2" diameter.  

I don't know how to calculate the size of the opening needed in the baffle wall, but looking at commercial smokers, this opening is not that large.  The 24 sq in opening seems to work for me. The opening length of 8" is about half of my firebox width. If my fire box was bigger, I might have made the opening longer proportionally by a few inches.   

I would love to see a real method to calculate the opening size if you find one.