I seem to recall that they are 18" black pipe. I'm going to remove the heat defuser first to see if it fixes my draft problem. It is large, and I can see it really slowing the air flow. If it doesn't fix the problem, I'll be looking at reworking the inlets. I'll probably go to using the elbows on the back only on really windy days. But I think without the defuser and the elbows off the back, it will have sufficient air flow. I'll be practicing with a chicken again this weekend; I'll post the results.
dp ~~ UDS, Blue Thermapen, Maverick ET-732