Can use 2x4s placed flat - so the 3 1/2" side is up flat for them to sit on. Can round the square top edges a little.
When it is cold, they can cover their feet with tummy feathers this way.
For short roosts, 2x2s can be used.
They end up walking up and down the roosts a lot, feeling out just the right position. They would slip a lot on round roosts and fall off. Branches off trees about 3 inches in diameter might work if they can walk them lengthwise. Bark woulld help with grip.
For chicks, I set up little 1x2s for them.
How you arrange them depends on your coop and birds.
My first flock fought to get the seat in front of the window with view. My second flock fought for the seat as far back in the corner away from the window. Birds are weird. This flock is still ignoring the roosts inside and sleeping outside on the sunporch (covered with hardware wire).
I have two rows at the same height. The birds behind can tuck their heads inbetween the front row birds. They seem to like to do this, especially in the winter to keep their heads warm, or just cuddling. So the boards are about eight to ten inches apart.
With your headroom, maybe one foot off the floor? They like to stand and stretch once in a while on the roost. Mine would climb over each other to get just the right spot too which takes some headroom.
Well then!This is horrible advice. The roost should be a size the chickens can wrap their toes around. I have a 1 1/4" wood closr pole for mine. If the chickens are forced to have their feet flat all the time its bad for them. Picture a parrot or parakeet cage. They dont have big flat roosts. The chickens are the same. They can roost on big 3" tree branches but if something smaller is availabe they would choose that over the larger rosst