Mine jump to get to the roost, so need a bit of head room and wing space to land. A reason to not crowd the roosting rods.
Also remember the length of the bird while sitting on the roost, enough space in front and in back of the roost. Don't know what size birds you have. I like minimun=m of 10" off the back and 14-16" to jump up. Though chickens are very able and 14-16 is generous. So hopping up requires the most space to accomplish getting on the roost; then they like to squish together and take up far less room.