Hi, I think it depends on the breed as in Small like Seabrights or Silkies vs. heavy bodied like Rhode Island Reds or Orphingtons. Most will say that a 2" x 4" on the flat is great , especially for bigger girls and especially in colder climates. A wider roost gives them area to squat down over their feet to keep them warm. Round, natural branches are widely used as well. Some folks here are afraid they can harbor mites due to their rough texture. I use a 1'' x 3" on the flat with a 2" x 4" on edge under it for strength, however I am spanning 12'. My girls have no problem and seem to like the width. So, thats my 2cts worth on roosts or perches. Good luck with what ya decide to use.
erik