The rule of thumb is 4 square feet per bird minimum, if they have access to an outdoor run. If there is no outdoor run you should aim closer to 10 square feet per bird. This larger number is also better if you live in a very cold northern climate where there are days or weeks that they will need to stay inside for extended times. Of course these are just rule of thumb numbers, and many people have done well with smaller or larger spaces. If the coop is too small they will tend to peck at each other and generally have issues being content. I agree with Bulldog6's reply, for a 4x8 you wouldn't want to go much more than 6 or 8 birds. And give them about 10" to 12" of roosting space per bird.