I know rule of thumb is 4 square feet per bird. Is that interior space or combined space? I have 6 full size birds (and 5 4 week olds) in an 18ish square foot coop. I never have to change the shavings on the side opposite the roost because they never use that side. They literally enter to roost and use the next box, even on 0 degree blizzard conditions, they prefer to be outside if it's day. The run is predator proof, so they have access to it 24/7. Our run is about 100 square feet, mostly precipitation free (covered in a tarp). They also get supervised free ranging a few times a week. I have no problems with pecking/aggression. Though, the rooster has a favorite hen. I am asking because of the chicks... I will be keeping the pullets (and cockerels until slaughter). Not sure who is who yet. I need an addition... How much bigger?? Here is the coop before I added the run a few years ago. The run goes to the left of the coop about 10 feet (and underneath) and is deeper than coop (total depth about 8 feet) and area under coop (16).