If you provide the minimum of 4 square feet per bird and the entire inside space is open to the birds ( no feed or water station taking up space, area beneath roost is protected from droppings to make it livable space, etc) 6x6=36/4=9 standard fowl.
That betting said there are many variables that could cause that number to not work. What is your climate/ location?
The word "could" makes that an interesting question. Commercial operations have proven that if you want to manage them a certain way (Clip their beaks to keep them from eating each other, keep breeds or hybrids specially bred to take confinement well, micromanage diet and lights, not mix ages or sexes, work your butt off managing poop) you can keep as many as 18 in there. Personally I would not do it.
You are in Pennsylvania so you will deal with winter weather. If you set up a fairly large outside area that is available when they are awake every day of the year regardless of weather si they basically only use the coop for sleeping and laying you can keep a fair number. I don't look at coop size in isolation, I see the coop and run as your space system and how you manage them has a huge influence in how efficient it is.
I cannot give you a number without having a lot more information. If you follow the link in my signature you can see some of the factors I'd consider. I find the more I crowd them the more behavioral problems I have to deal with, the harder I have to work, and the less flexibility I have to deal with any issues that come up. Most of this stuff is more for my comfort and convenience than specifically for the flock, but they lead to less stress in the flock too. I don't like a lot of drama and undue stress in my life. The extra flexibility some extra space gives you so you can handle emergencies is very important to me.
I've seen recommendations spanning form 1 to 15 square feet per chicken in the coop on this forum. You can sure get a lot of different opinions. In my opinion the 4 square feet per chicken in the coop if you have a small backyard flock that is all females the same age and raised together is not a bad starting point. The more outside room you can add to that and the more time you can make it available the better. But if you ever want to integrate new chickens and especially chicks, either broody or brooder raised, it will get really tight. If you even need to isolate a hen either to heal from an injury or to break her from being broody, where would you do that? These are probably some of the things OGM is thinking of when she says 4 square feet doesn't always work.
Ok, thank you. We have 7 chicks right now all being raised together. I was debating adding another 4 feet to the coop but I wasn’t sure if the smaller size would be better for our winters.
So, I guess for now I should be ok. (Unless we get more chicks). Lol
/Forgot to mention that we have a 7x12 run for them atm also
I've "read" 4 sqft per for the coop & 10 soft per for the run. However we all know BIGGER the better = Chicken Math as you are experiencing If I didn't live in a residential area & had space, my coop would be HUGE