I am going to name everything I can think of so bear with me!
Chickens require 4 square feet each. You should include space for a feed station. If the coop holds 10 chickens, I'd put 9. Have a seperate coop, or whatever for quarentining birds. This is necessary for biosecurity. I strongly suggest a coop you can walk into. This is why we are building a new coop. If you want just a few chicks, then maybe something like the playhouse coop would be ideal, but if you're going for 10 or more I would definitely press for a walk in coop. If you look at the top of the page at the coop designs link, click that, there is TONS of information. Make sure it is predator proof. If you are getting standards, then you should be able to get away with 1x3 welded wire. It is strong and durable, although a pain to put up. You may also consider buying a dog kennel, and placing chicken wire or hardware mesh around the bottom to prevent them sticking out their heads. VENTILATION VENTILATION VENTILATION, NOT DRAFTS. It gets humid and moldy fast if you don't have ventilation. Runs need to have 10sq feet per chicken.
I don't think we will be doing more than 10, but who knows what will happen down the road.
We live on a 1/2 farm lot so they will definitely be free ranged.
I used to keep bees (developed an allergy to the sting) and the boys will be fixing up the area for the chickens, though when we are outside they will have more room to roam.
J-P, my son who is designing the coop is very excited. He loves to build things. But hubby said they have to pay for it themselves so it should be interesting.
We (hubby and I) were thinking that maybe we could add a tarped section in front of the coop in the winter. Giving the birds more room to run around and access to ground. Plus we could put a dust bath there too.
Remember, this information is from people with many different goals and restrictions in keeping chickens. Some of us raise them for meat and/or eggs, some as pets, some to win grand championships at shows, some as part of breeding projects. Climate plays a part. Are you raising banties or full sized? How much room are you going to devote to them; that is size of run or free range. Each of us have differing circumstances and we have different opinions on a lot of things. That does not make any of us right or wrong.
I'll give an example. Omran is right that the general rule is 4 square feet per chicken. However, there are certain circumstances where you can get by with less. If you feed and water outside, have nesting boxes outside, have lots of room for them to roam and shelter from weather outside where the coop is basically just a secure roost, you don't need as much room. There are also circumstances where you need a lot more. If the chickens need to spend weeks at a time inside the coop due to climate, 4 square feet per chicken will not be enough.
You are on the right track, doing planning up front. Keeping chickens doesn't have to be hard. People have been doing it for thousands of years. Chickens are pretty hardy. If you get the basics right, they'll do fine.
When I get my big coop and run built, I was going to make a dust bath. If you don't make one, they will! Mine like to do it right by the edge of a building, I think it's colder dirt. I was going to take a kiddy wading pool, the little plastic ones, and fill with sand, DE, dirt and whatever else I can get for dust baths. Right now they have a litterbox with sand in it, but it's not big enough