Is that the total room you want to give them or is it only the coop and you have more room in an attached run? I'm not a believer in magic numbers for space, you can follow the link in my signature to see why. But that 4 sq ft per bird in the coop is usually associated with an additional 10 sq ft per bird in the run. Chickens do not recognize coop space versus run space, they just see space when they need it. A run doesn't do any good if they can't get to it when they need the extra room.
That 4 + 10 are pretty good guidelines for most of us. For some people it is plenty, but occasionally it can get tight. I yours are all adults and already integrated it should be enough, especially if it is s time of the year when they can get outside practically all day every day. If you were integrating or having a broody hen raise chicks it could be pretty tight.
I'd think a 6 x 8 coop plus an 8 x 12 or 8 x 16 run would do quite well for you. A 6 x 8 pen by itself is probably asking for trouble. And if you use it in better weather it doesn't have to be much of a coop, just provide protection from rain and predators. In winter you need to do better where you are.