I agree with Pat, above. Don't get tied to a square foot number-- chicken management has to take into account so many more factors!
My flock of 25 birds are in a 6 x 8 coop (so even smaller than yours!) and are doing great. They free range, and are only in there to sleep and lay eggs. We get plenty of rain, cold temperatures, and snow several times a year, and my flock chooses to be out in just about all of it. The only time they choose to stay in is when there's a serious amount of snow on the ground, and then they seem to happily hang out in the coop, on the roosts, and eat me out of house and home. No scuffles, no problems.
So watch your birds carefully, know when they're happy and when something's out of sorts, and you'll do fine, however many birds you choose to keep in your coop.