Hi Denali.
I'd encourage you to plan it right and build it right. Why? Because you are going to love having chickens, and you'll really love going out there and admiring your good handiwork if you take my approach.
The first question I'd have for you is Winter: how bad are yours. If they are bitter cold, then you can push that 1hen = 4 sq ft rule, up to 1 hen=10sq ft. That way they have room to mull about during periods of rain and snow.
Next, why not build a rectangular building that is a dividable by 4ft. That way, you have less waste and cutting. Let's just say you build a 200 sq ft bldg, for example: consider this: how about a 12x16' bldg. That's 192 sq ft, and close enough to 200. This way, you'll have little waste. If you use plywood construction, your one dimension will be 3 sheet wide, and the other will be 4 wide.
Keep in mind it's handy to have an indoor storage area where you'll keep the feed. You'll also have a roost area. With good size coops/flocks, there will be a fair amount of poop generated. If you plan it right, you can have an exterior door that opens to underneath the roosting area. One strategy is to put some chicken wire right under the roost boards, and around the base (to keep hens out), and then just have a board below (like some scrap plywood). Or, vinyl. Once you have enough poop, just sort of shovel it off and store it elsewhere. This can be used months later for excellent fertilizer, and you'll never have to buy any again.
Good luck.
Brian