I'm not sure where you live. People in cold climates like to use 2x4's with the flat side up so the chickens settle on their feet at night and keep them warm. Many others use a 2x4 with the thin side up in warmer climates. In both cases, the corners should be rounded.
Chicken's ancestors naturally roosted on tree branches. I'd suggest an inch and a half minimum diameter and larger is probably better. If you use a tree branch, you need to take the bark off. Roost mites hide in cracks during the day and attack chickens at night while they roost. The bark gives them a place to hide and makes it hard to treat.
I don't have pictures but I made mine removeable so I can clean the coop easier. I drilled a hole through each end and in the support it is resting on, then put a nail through to hold it in place. Or perhaps you could notch the support to receive it.
I'd make it a minimum of 6 feet long, at least 1 foot from the wall, and about 12" higher than your nesting boxes. Do not put waterers or feeders under the roosts. Use wood. Plastic or metal cools off too much and is too hard on their feet.