I will need to go out later to get exact measurements of the coop. The chicken run is converted from an old horse run-in. It's about 13 feet by 24 feet, and 12 feet of the length are covered by a sloped metal roof. So half of their run---the half where they roost---is enclosed on three sides by wood walls and then the metal roof on top. (There are triangles of ventilation that I have covered with hardware cloth). At this point, their roost is a large tree branch that I got from the woods and nailed diagonally into one of the corners.
The coop is a pre-fab, and at this point it would not fit my whole flock. (I had 2 or 3 medium sized chickens when I bought it. I now have 6 chickens in that area. I remember that the coop said it was rated for 6-8 chickens and that was obviously a total lie.)
And I do very much trust my vet (well, former vet. She's now doing international consultations and out of the mobile visit business. Her credentials are here:
https://www.vin.com/vin/default.aspx?pId=130&id=8879519&meta=108). She had a lot to tell me about my set-up in terms of the health of the flock, predator safety, etc. She said her main concern about chickens sleeping outside of the coop were being safe from predators and not being too exposed to wind when the weather gets very cold. The run is safe predator wise, and if there's a bad cold snap I can hang a tarp over the "open" side of the run to cut down on the wind, so she said not to worry too much about them not being inside the coop at night.
I've been intimidated at the idea of building my own coop, but my sister and brother-in-law recently built their own and it's a lovely design with lots of space on the inside. One of my goals this summer is to see if I can get them to help me build one for my girls. It is tempting to have that extra layer of safety and protection from the elements.