The more room the better...... for the chooks as well as your own ease for maintenance and having some flexibility to deal with new chicks, broody hens, injured birds, rogue cockerels, etc. Think long term. I built my coop with 2 people doors to facilitate splitting the coop with a temporary chicken wire wall, and later added another pop door and separate run. Was the best planning decision I made, it has been invaluable.... and I wish I had one or two more partitions. Really made new crisis' during first year much easier to deal with, rogue cockerel spent my first winter in there while I figured how to harvest him in the spring. Then chicks in there, then a broody hen. I take down temp wall in winter for more room as they spend way more time inside coop during winter where I live. Check out My Coop page, link under my avatar, for details and pics. Another thing about space that is often not thought about is the height of the coop. Being able to walk into coop is a lifesaver for me, bending over or crouching/kneeling down is not tolerated well at all by this old arthritic body. I built my nests and roost boards at a height easy for me to work at.....and they work good for the birds too, but needed ramps to balance heights with width of coop. I was lucky in that the existing building my coop is inside of is very tall with a clerestory roof line, the high windows and open eaves make ventilation a breeze(haha!). There's a 'stack up' involved when planning coop...I should draw up an illustrative sketch for this, maybe today...but....I digress: Bottom of poop door above highest bedding planned...so bedding doesn't end up outside coop. Bottom of nests, if hanging off wall, high enough so floor space underneath is usable by birds with highest bedding height. Roosts 12" above nests so they don't roost(sleep) in nests and poop them up. Upper ventilation, especially in severe winters as well as in very hot climates, as far above roosts as possible.