I think most of the answers you are getting are spot on. I will add one thing, or more appropriately expound on one point.
As a relatively new chicken owner, I did a pretty good job of designing and building my first coop. Mine sounds a lot like everyone's. It is a 4x6, with 4ft walls, and is a bit higher in the middle with the gabled roof. Their door is on the south side (a long side) leading into their original run. I even rigged a lightweight chain over a couple of nails to raise and lower their door without going into the coop. Mom lets them out in the morning, so I wanted her to be able to take care of such things without tripping over them, or possibly dealing with an aggressive roo. The west side has a triple nest box attached from the outside, with a lid. I agree that that is an important consideration. The other short side on the east is a plain wall. The north side has a 2ft wide door so that I can clean out the coop and get inside for repairs and such. To either side of it, is a 1ft square window with 1/4" hardware cloth. Just like with their door, I cut them out with a jigsaw, and simply added a hinge and latches to the cutout and put them back in place. My coop is about 30" high at the floor, perfect to pull a wheelbarrow right up for cleaning. Its also high enough for the full grown roosters to walk around underneath. There are two main roosts running along the 6ft sides, set inside about 8", and are 2x4s wide side up. They are about 3ft off the floor. There is a single bar, narrow side up, running right down the middle, and about 15" off the floor. This is mainly so they can jump onto it, then jump to the main roosts. For my girls, they seldom even flap to jump anywhere in the coop, which they routinely do.
The point I wanted to bring up is lighting. I am not a big fan of having to run electricity to the coop. I have learned, though, to make sure you take into account that your chickens will get adequate light before roosting time. Apparently they dont see well in the dark. Jumping up to a roost in the dark isn't easy, hence they will usually be on the roost before dark. They will get fussy over favored roosting spots, and occasionally someone winds up on the floor as it gets dark. You will see quite a few posts on here about hens sleeping in the nest boxes... a hen knocked off the roost with it too dark will find somewhere to feel safe during the night, usually the nest. I don't think they need an electric light on a timer, they just need natural light as long as possible. I never had this problem until it started getting cold enough to close the windows up during the day. The first day I had the windows closed, I was amused t watch my adult rooster Vic walk into the coop at roosting time, only to walk right back with a puzzled look. He walked back in, then poked his head out and looked at the sky. It was light out, but dark in. I opened one of the windows, and he promptly found him a spot to roost. I also noticed that when I found one of the girls in the nest after dark, shining a flashlight inside would prompt her to back to the roost. Last night, before the snow hit, I found 2 still walking around in the run an hour after dark. Same thing. I gave them a lighted way into the coop, and up they went.
I may add some plexiglas panels for the windows the next time it warms up. That way I can leave the windows closed for warmth but give them enough light. For the time being, I did put a tap light in the coop that I can turn off from the outside via the nest box. I can leave the windows closed on really cold days, and turn the tap light on just before roosting, and turn it off when I lock them up without disturbing them. I hang out with them at the big door while they are fussing over roosts, and lock them up after they settle down.
I'll add a few pics of my set up. Don't despair over coop design. The only flights of fancy in coop design should be your own personal aesthetics. Mine is simple 2x4 frame covered with exterior grade OSB sheathing. The shingles were leftover from roofing the house many years ago. I have the linoleum to put on the floor, but after 6 months still haven't done it, but no problems. I use straw for bedding. Every few days a take a short rake with long tines and shake around in the straw. Nearly all of the poop falls through to the bottom, and once every couple of months, or more if it noticeably starts smelling, I rake all of it out and put down new bedding. Their food and water is set on bricks under the coop to keep the rain off. So far, I haven't had any real issues, other than the lighting. I don't even have poop boards.
Here is the set up of the roosts, you can see their door on the opposite side.
Here are my girls hiding inside from the snow. My 4x6 coop is great for keeping them cozy in cold weather, but I really need more space under a roof or awning so they can be outside and out of the weather. It houses 11 RIR hens and a RIR cockerel (all nearly 6 months), and an adult cross bred rooster. Great for roosting, but I need to hurry up and cover the original run.
Here you can see the cockerel, Roo, with a couple of the ladies. He walks around with his head over 2ft tall, and Vic is bigger. You can see they have plenty headroom.