In calculating your coop size, do NOT count on the idea that the chickens will use an uncovered, outside run (or be willing to free range) in the snowy winter.
Chickens will peck foam insulation. Yes, you do need interior walls (preferably not sheet rock which is susceptible to moisture).
Chickens will somehow manage to splatter poop just about everywhere.
It is very handy to have your waterer in a location where poop splattering is minimized. My waterer is at the end of a narrow "shelf" - they just go up there to drink.
Plan out where you will keep supplies for the coop:
food, treats, poop bucket, pine shavings, cleaning tools (dustpan, old car snow brush, kitty litter scoop, broom), stall dry, DE, chicken care items like petroleum jelly, grit, oyster shell.