I went with a 12' x 12' for a dozen layers a few years back. The run is my kids' old play yard which was 50' x 50' before a few additions and adaptions. I like my critters to have lots of room.
If I knew I was going to be getting into different breeds and bantams (4-H and chicken math) I would have built a divided coop and runs, back then. But I have lots of room for expansion and an understanding husband.
As they get older they should have at least 1 sq ft of space per bird, less if they have a run or free range. I have 15 chickens in a coop that is only 5ft by 7ft so it doesnt have to be too big. 8 chickens, you should have a 5ft by 4ft coop at least, but always over estimate cause you probably will catch chicken fever like the rest of us and get more.
Mine is also a 12x12 but it is divided for geese one side chickens can go either siide but chose not to for there own good, but do go up to the loft area up top also geese go out all day. Chicken go out half the day in winter all day in sspring and summer.
Not nearly big enough!!! We have a 4'x12' and 14 girls. They are fine in it most of the time since they also have a dog kennel run and free range over an acre. It does limit us getting more chicks so we found another home for a few of them to make room for new breeds.
1 square foot is absolutlely NOT enough space! A 4x8 foot coop would be 'sufficient' but an 8x8 coop would give you room for storage or more chickens in the future...trust me, you will want more. If you live where you can only have a certain number then 8x8 is still a good size. Allow 10 sq feet per bird in your run. More room means happier healthier chickens.