I'm getting closer to finishing the mortaring. Here is the chimney and flues. One mistake I made was that I did not tie in the first layer of bricks that sit atop the flue brick into the walls of the kiln. I say "Just deal with it."
The measurements at this point are: inside is 23 inches wide and 18 inches tall. The chimney is 8 inches deep and 23 inches wide. This is the point where I am going to start tapering my chimney to get to a 9 by 9 inch hole.
The flues are 7 inches tall. The center flue is 2.5 inches wide and the others are 1-3/8 inches. I didn't follow any principles from any books regarding flue size and intake holes. I just built them based on photos I've seen of other kilns and figured better too big than too small.
Today, if the rain holds off, I hope to finish the tapered chimney and the front of the kiln.