Found it. It said this:
“We cannot refute that
Rhode Island and Australorps are heavily-built species. Nevertheless, Reds are slightly larger with Rhode Island Reds adult roosters weigh roughly 6.8 to 8 pounds and hens at 6 to 7.5 pounds. More or less, Bantam Reds weigh 2 to 3 pounds. Inversely, the average size for Aussie roosters is 6 to 7 pounds, with the hens approximately weighing 5 to 7 pounds. In addition, bantams averagely weigh 1 to 2.4 pounds.”
So I still won’t know if a 12” space will be sufficient for one RIR. But it will be worth it to try cause if I can have all the roosting confined to the left side then we can build a nice sandbox poop board and make daily cleanup much easier.