Hi there! Our henhouse is 4x8... and we have 2 roosting bars in ours. I have 9 hens and a roo and all of them are up at once! It's pretty funny to see. They have Cliques-BFFs! But all 10 are up and comfy! When they hear me coming I can hear them jumping down one at a time to see what goodies I'm bringing!
For standard size chickens, use a 2x4 flat side up. It gives them plenty of foot room and they can squat and cover their feet when it is cold. Make sure the board is splinter-free. Some say to round the sides, but I didn't bother and my girls haven't complained yet.
You can use a tree branch, a 2X2, 2X3, 2X4 as mentioned above or just about anything that will support their whole foot. They will take to just about anything. They do not like anything slippery though. You need about 10'' of perch/roost for every full sized chicken.