I followed the 4 Square Foot Rule in the past, mostly because that was the average of all the modern wisdom. I built an ark according to that and I even wrote an article on the mathematics of the whole concept.
Then I began to read the archane texts, by guys like Kains and Hastings and Robinson. I learned that this whole 4 sq ft thing was an offshoot of modernistic commercial practice, which has filtered down into poultrydom at large. Since then I've dumped it. It is true that there are no absolutes here but you can come close - and should try.
So here it comes, the Revised Space Allowance guideline, or "RSA" for short:
"It is far better to have fewer chickens than it is to cram too many of them in to a small space without respite."
Now this will go against the grain for those whose nature it is to overdo things. If a BOGO sign (Buy One Get One...) in front of a store causes you to whip out your wallet and careen into the parking lot, be careful here! If your OTHER favorite website is FreeCycle, becuase you can get lots of "cool stuff" there, BEWARE! - - -
You will surely suffer from "chicken want-itis," which is simply wanting as many of the cute little darlings as you can get your clutching hands on. Unless you have planned for this you will do no one any good. This is where the old adage, "Know Thyself," will come in handy.
Barq has a small roost area, yes, but a chicken only needs about 18 square inches for that activity. I've had hens that fit 4 of themselves in a standard nest box and be happy that way. But day to day living is another matter. Think about the size of your bed, when compared to your house, for example.
Also note the admission the the birds go in the house only to lay and roost. The rest of the time they are out. Wise, wise, wise. Chickens need fresh air and lots of it. Their metabolic rate is high and they use a lot of oxygen.
Someone mentioned ammonia. When you confine them tightly, their droppings exude huge amounts of ammonia gas as they breakdown. This gets in the lungs and can cause all sorts of respiratory problems if not dealt with - again, waste management. The commercial poultry business has devised all sorts of methods to combat this. Near as I can tell, it is perhaps their primary concern, from which most other problems arise.
When I said chickens do heathenlike things to each other when crowded, I meant everything from disease to cannibalism stems from that one factor. I know I'm beating a dead horse here, but you can't "fudge" this basic element of poultry care. Pollyanna idealism wont help you when you fail in this matter.