You'll get different opinions from different people. In the brooder, anything from 1/2" wide to 2" wide will get used.
In a coop, many people use 2x4's with the corners rounded off. In cold climates, people often lay the 2x4's flat so the chicken will cover its feet while roosting, keeping the feet warmer. In warmer climates, that is not necessary. It might even be better for the chickens to cool off a little while roosting. Chickens handle cold better than heat.
You can also use tree branches for roosts, but remove any bark. Red mites hide in cracks and crevasses during the day and attack the chickens at night on the roosts. If you remove the bark, these mites have fewer places to hide and the roosts are easier to treat.
Do not use metal or plastic as they are too slick. The metal will cool off too fast also, which could cause frostbite problems in cold climates.