It sounds like you are talking about a small elevated coop. I don’t have one of those so I hope people with one see this and tell you how they laid theirs out.
You do not want the chickens pooping in your nests, feeders, or waterers from the roosts. That can be a challenge in a small coop. With mine the nests extend out into the coop with the juvenile roost over it. The way it is aligned the chickens cannot poop inside the nests from the roosts but as you can see they are pretty dirty. That doesn’t bother the chickens but it would some people. I have good access to clean them off, that’s on my agenda to do today.
With your nests hung off the side of your coop, that opening will probably be flush with the inside wall. I don’t know if the chickens will poop in that from the roosts or not. Probably not but I haven’t tried it. But if the bottom of those openings are too low, the chickens can scratch stuff from the floor of the coop into the nest. That stuff could include poop. So raise the openings into the nests a few inches above the top of any bedding you use. With five or six hens I’d suggest two nests. You might get by with one fairly large one but two would probably be better, especially if you go with the minimum recommended size of 12” x 12”. Mine are 16” x 16”, mainly because that was my stud spacing so framing them was easy. Even if you make them bigger, I still suggest two nests.
You’ll find all kinds of recommendations through this forum for how much roost length you need. Some will say 7” per chicken, some will say 15” with all numbers in between getting votes. For five chickens I think you will be OK with one 4’ long roost. For six chickens you might be Ok with one 4’ roost, you might not. If you put one roost the 6’ length where do you put food and water? There are a lot of variables involved, including the individual personalities of your chickens. If you fool with them much you’ll find that each chicken has its own personality. That makes it a little harder to determine exactly what you need when you go small or tight. There is some trial and error involved.