Generally we recommend 4 square feet of floor space per bird in the coop, plus as much outside as possible. You can probably manage with the space you are constructing, but watch carefully for respiratory problems. You can avoid most of these problems by being very aware of the conditions where the birds are, particularly at night. You will need to clean your building frequently so as to avoid putting a lot of ammonia and other irritants into their air. Congratulations on leaving at least part of the rat race.
Chickens are a wonderful way to help get reconnected with reality.
Good luck to you. I think we can add another4x8 sheet to the floor later. According to this standard, we do still need to add. As for ventilation, we are planning on using an exhaust fan in the warmer weather, giving daily attention to ensure it does not become blocked and there are two smaller vents at the roofline opposite the fan and a window that can be opened as needed. We really can't get anymore outside space as we live on a third of an acre in town with close neighors to appease.