2x2 with beveled/rounded edges.
they are birds. nature designed them to roost on branches. Branches grow round in nature. Don't try to out-think mother nature.
True, branches grow round. But branches can be any width from paper-thin to more than a foot wide (which is effectively flat for a chicken's feet).
If anyone has chickens that do roost on trees, it would be interesting to know what size branches they choose to be on, and whether they chose the smallest or largest branches on the tree (as compared with choosing something in the middle.)