By far my favorite design is a simple shed, at least 8x8 and walk-in, with a roof that slants only one direction. This makes it very easy to ventilate it well by leaving an area open between the roof and top of the wall on the high side. If you are in a snow area, you can slant it enough for the snow to drop off. If it is a hot climate, you can leave it open all the way around the tops of the walls, which is what I did in this warm climate, though we also made parts of 2 walls out of hardware cloth for breeze.
I love having enough space for a garbage can for feed and a shelf or cabinet for supplies. We also have about a 5x5 "coop in the coop" with chicken wire walls for injured birds, a broody, new chicks, etc., but this is in an 11x17 coop. I also don't care for external nest boxes. They can cause leakage and security problems, but mostly, I want to check on the birds anyway. Besides, someone sometimes lays an egg in a corner instead of the nest. I've had chickens in this coop for 3 or 4 years and wouldn't change a thing. I've had other coops over the years. They were all big enough to walk into and store feed in. If you can, running electricity and water, or at least water, to the coop is well worth the trouble, and not really that hard to do or expensive if you can do it yourself. Modern pipe is pretty easy to work with.
Just my viewpoint, of course. It is a dirt floor cop, by the way, because I wouldn't have another kind.