Usually cook a 13-15 pounder in my weber 26.75 inch kettle with the smoke-n-sear taking up aboyt 30% of the space on one side. How big of a bird can I fit on the other side, might want to go bigger?
Probably depends if you are spatchcocking or not. If you are leaving it whole there probably isn't a bird that won't fit. I've cooked birds up to 20lbs on my 26 without problem but I don't have a SnS. Even still, I can't imagine you wouldn't have enough space for a big bird.
Once you hit 18-20lb the breasts tend to overweigh the rest of the bird. Fitment aside, you're usually better off with two smaller birds. You now have twice as many drums, thighs, wings. If you're serving a 20lb turkey then you have 8-10 people who I promise will be happier with some more dark meat pieces