I can't resist chiming in. Six years ago, I began with two adopted adult hens. I built a small coop, 4'X4'x 3' tall, two feet off the ground with a small four by eight foot run four feet high. It was adequate at the time, and was still adequate when a few months later I acquired three month old chicks.
Six years later, my coop and runs have grown to 1700 square feet with 21 chickens. That's the result of adding on four or five times over those years, tearing down runs and building them higher and larger and adding onto the coop and building a second one.
The biggest mistake I made was not to make my coops large and tall enough for me to walk upright into. I thought the large access doors would make it easy to reach inside to clean, but I still need to get inside to do stuff, and am always bumping my head. Also, it was hell, adding onto existing coops, as they never square up, and it's really hard to do.
So my advice is, as long as you can afford it, to build it tall, and build it big enough to surpass what you eventually dream of having in the way of your ultimate flock.
