depending on where you are from will make a difference, , but planning ahead if your flock expands is a good idea too, I built mine but it is super easy to add on to as needed too but I doubt unless I move will need to or chicken math would really catch up to me lol.
Well, 10 sf is a recommended MINIMUM, but honestly, I look at my 120 sf run, and try to imagine 12 birds in there, and can't. Go as big as is practical, based on your space and your wallet. Love that you are going bigger with your coop.
4/10 is a minimum, IMO.
Many factors determine what space is best your your goals and conditions.
Great articles on Space and Ventilation linked in my signature.
Go bigger......and.....let us know.
Where in this world are you located?
Climate, and time of year, is almost always a factor.
Please add your general geographical location to your profile.
It's easy to do, (laptop version shown), then it's always there!
I was out with my chickens today, thinking about this very thing. I have 4 standard hens, 1 standard pullet, and 3 three-month-old bantams. They have over 200 sq ft in the run, and it still feels cramped when they're all in there. And I live where the coldest it gets are 40s and it never snows. Go as big as you can afford, imo.