That topic has been discussed in another thread here, and one fellow commented that he heard that the correct firebox size is 30% of the smoke chamber size.
Another said this:
I just finished one that is about the same size. It over smokes the meat. Mine firebox is 24X24X24 and has a hard time maintaining a consistant temp without a huge fire. I believe 18X18X24 OR 16X16X24 is more than sufficent. I will let you know how mine works after the modification.
