The chute is a product of what we had laying around. I though the 6 inch square was a little light and could have burned out over time. Plus I wanted a little bigger tube in the burn area. I'm glad I did. I think I would have had trouble getting this size pit up to temp with just 6 inches square burning. This size is very close to the Stumps platinum line, and he tapers the chute at the bottom on those pits. I'm not sure the dimensions though.
As far as the exhaust goes, the suggestion I keep getting is to make it a little bigger than you think you need. It can always be baffled down, but it is hard to make it bigger.