Rule of thumb I was told is min 2 sq ft per bird in a coop and 4 sq ft for a run. I have found this to be far undersized.
My coop is about 12 sq ft, and while it hold my four hens nicely, six will be rather snug. My run is 24 sq ft, and is way undersized for my four hens, even after giving away the aggressive one. I am planning on extending the run to be 20' by 7' so I can increase my flock to 6 birds, and I can only get away with that by letting them out into the yard to forage for a few hours each evening, breaking local ordinance.
My advice is to go big. Just like with decks and patios(I'm a carpenter, and I build those), I have never heard a customer complain about their deck being too big, but they have complained about it being too small. Make your coop much bigger than you think you need. And if it turns out you have more than enough room for your hens, then you have room for more. And we all know the number you start out with will not be enough, and next spring you will be getting more.
