FaykokoWV makes an excellent point. Are you going to want to walk in it? An 8 x 5 would be tight with you, nest boxes, roosts, and maybe feeders and waterers in it. I'd suggest making the roosts removable if you are planning on working inside. If not, make sure you have plenty of access doors and hatches to clean it.
Aside from the walk in question, the bottom of the nest boxes need to be about 12" above the top of the litter (not the floor) so the chickens can get under them. This way the nest boxes do not count against your floor space. If you cut this too close, the chickens will scratch the litter in the middle of the floor under the nests and not go under the nests and clean it back out. You wind up with a lot of your litter piled under your nest boxes. That litter stays pretty clean but the middle of your floor gets pretty bare unless you keep adding litter.
Your roosts then need to be noticably higher than your nest boxes or anything else you don't want them to roost on. If the top of the nest boxes and the roosts are close together, 6" is probably enough. If they are on the opposite sides of your coop, maybe 12" would be better.
For good ventilation without getting your roosting birds in a draft, you need enough height above the roosting birds to put your vents so the cross draft does not hit them. With a walk-in coop that kind of takes care of itself. With the smalller ones, it is something you should consider.
FaykokoWV also makes a good point about them possibly hurting them selves jumping down. The heavier the breed, the more likely that is to happen, so you want the roosts to be as low as possible. Another reason to have the roosts as low as possible is that the chickens flap their wings and fly down when they come off the roosts in the morning. They are pretty clumsy flyers and can hit feeders, waterers, nest boxes or walls on the way down and injure themselves. The higher the roost, the more horizontal clear room they need to come down. My roosts are just under 4 feet high and most of mine use the top of the nest box to get on the roost, but they all fly straight to the ground. So I suggest making the roosts as low as you can.
I don't know what your final layout will be. With 10 chickens yuo need about 8" minimum for roost length, so one roost across the 5 foot width will not be enough but one the 8 foot length will be. My set-up is different from yours, but I put my nest boxes on a wall perpendicular to the roosts. I'll throw that out as a suggestion for you to consider. As you can see from this photo, I did not follow all of my advice when building mine, such as the height of the nests off the litter and keeping the height of the roosts as low as I could. Should we say that experience is a good teacher.
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