Mark - I read Nils Lou's book "The Art of Firing" just before building the chimney, and he suggests putting one course of bricks in the stack, a few feet above the flue, that reduces the size of the inside, in order to create turbulence. I was under the impression that my chimney was way over-sized at the time, and thought this layer would possibly help, and if it turned out the chimney was far too wide, I could slip a smaller pipe down to sit on this layer if need be.
I didn't measure the interior dimensions of the hole when I built it, but it appears from photos to be about 6" x 6" or maybe a tad bigger. http://kgspottery.co...gAnthony/43.jpg It's about 2' up from the base of the exit flue. The rest of the chimney, both below and above this one layer, has a 9" x 9" space.
p.s. The second half of my name sounds funny when applied to me. "Gail" alone is my mum!