I use a 2X2 with the corners eased a bit... My chickens like this set up. And I know this cause they told me so!
Seriously... they'll use whatever you put in there, although they do prefer about a 3" round Stick, log, dowel, whatever!
No offense to the 2" up-siders, but everyone should have a 4" flat for the birds to roost on.
I made a new perch this week, raised quite high so I could get a good view of them perching ~
with a four inch flat side not only can they sit on their feet to keep them warm, they can also keep their legs warm.