Our coop pages (brown strip, top of page) have lots of coops of different sizes, many with good plans.
I have an 11x17 coop with a dirt floor. There is a "room" in one corner, about 5'x6', that works well for a broody or new chicks --- or a sick chicken, etc. The "walls" of this smaller room are chicken wire. Rather than have a separate storage room, I went with two galvanized garbage cans. One holds 100 lbs. of feed and the other is where I keep oyster shell, grit, BOSS, and anything else I don't want the chickens to get into. I have sometimes wished I had a small cabinet up hgh on a wall (so they couldn't roost on top of it) to store small items such as scissors to open feed bags, a bottle of BluKote, and so forth. I prefer the garbage cans as they are near the feeders so it's easy to fill the feeders. I have running water just outside the coop and power wired to two outlets in the ceiling, which makes it easy to use the small chicken wire room for a brooder. The people door, several inches between the top of the wall and the roof, and half of the wall opposite the people door are all made of hardware cloth. This airs out the coop so well that in the summer, they will go in the coop because it is cooler than under the large bushes intheir yard -- even though it is a metal building. The roosts are in the corner next to the half hardware cloth wall. To prevent wind blowing on the roost in winter, I staple a couple of plastic bags over the hardware cloth of the people door. Other than the small cabinet, there really isn't anything I would change about the setup. Well, maybe a removable panel for the people door, just for looks. I love my dirt floor. With 9 hens and pine shavings, there is no odor and the litter is only removed and replaced once a year. A lot of people like sand, but keep in mind they scoop the poop out of it.
If it's big enough, the floor plan won't be that much of an issue, as it will be easy to change things around. Themain thing you will read here is, build as big as you can. I would not keep chickens if I didn't have a comfortably sized, walkin coop -- maybe not this big, but for even 9 hens, I wouldn't want less than 8x8. Because lumber comes in multiples of 8, it's much easier and more practical to make it in multiples. For example, I wouldn't build 10x10, I'd build 8x12. Same square footage (almost) and lots less cutting. A roof that slants to one side only gives much better ventilation, too, as the warm, humidified air moves out best if from the high point of the coop -- and a vent on a peaked roof is a pain to build. for the same reason, among others, I would never consider outside nest boxes built onto the wall. I want to go inside and check on things, anyway, including checking for eggs that weren't laid in the nest box. Outside nest boxes are a pain to build, tend to leak, and are difficult to predator proof. With my coop, I can use any handy container in that size range for a nest box. In a pinch, a cardboard box works fine.
If the barn is bigger than you need for the horses, you could also consider converting a stall to a coop, with hardware cloth around the top edges for good ventilation. Lots of folks have done this.
Good luck! Hope some of this gives you some ideas.
