I asked a while back about roost space and the unanimous consensus was that no matter how much roost you provide, they always use less. We'll have 10' for 12 chickens but could add a little more if we have to. In our current coop we have 6' and they all fit but it's looking very crowded lately.
Roost space is always important. Figure about 12" roost width per chicken. No, they won't use all of that all the time, but there is always squabbling and jockeying for position at night as the flock settles in. If the girls are crowded, then the squabbling could turn into serious pecking/bloodshed which means problems for you as the chicken keeper.
Floor space is also important if your flock will ever be confined inside the coop during daytime in periods of bad weather (severe winter, for example). But if you live someplace where the flock will be able to go outside every day during daylight hours (and you'll always be there to let them out in the morning, or have an automatic door), then floor space inside the coop is less important.
It's usually suggested to allow for 1 nestbox for every 3 or 4 hens, but they do like to lay in a single nest no matter how many you put in.
I have 40 square feet of coop, 6 feet tall at the tallest, 15X15 run and a 4X8 sand box under the coop for 7 hens and a rooster. Whenever that take a chill out minute that's where they go. Matter of fact that's where they spend most of their time. The feeder is outside so all the eating is outside. So far mine seem content, that is except for one of the RIR's thinking she's the boss. Just one of um does all the chasing.
If you ask me, I'll say floor space. If you ask my chickens, they will say roost space. My chickens DO NOT like being crowded on the roost for some reason. They hate it. They will peck the next chicken until that chicken scoots away from them leaving them some space between the 2 of them. I have no idea why all of them do it.