My 2 cents...
I always plan 4 sq ft INSIDE space per bird. For 8 birds that's 32 sq feet minimum, round up for easier math and cutting, 4' by 10' is 40 sq ft...4' by 12' would be a sheet and a half of plywood (easy cutting and leaves plenty of room for "chicken math") and leaves room for 12 birds at maximum capacity, meaning plenty of room for the 6-8.
Then, you have OUTSIDE space...doesn't include what they have inside at all...10 sq ft minimum outside....so you need at least 80 sq ft for 8 birds....going off the"long side" of a 4' by 12' coop, you'd need to go 6.5-7' out to provide minimum space, again though, go BIG for chicken math and happier birds! I'd go 16-24' out for easier cutting (8' lengths times 2-3 of them)...this would provide 192-288 sq ft of outside space...or 24-36 sq ft of outdoor space for 8 birds.
My original flock was only 3 hens, all I could legally have at the time. They had a 4'x8' coop (pallet crates turned upside down on a 4'x8' floor), roughly 5' of roost inside and was raised up on pillars so they could also use the space under the coop. The exterior area, including under the coop, is/was 12'x15' (the underside of my second story back deck), giving them 180 sq ft of outdoor space (60 sq ft per bird).
Since I don't free range at all (too many dangers for them), I want to keep the space as large as possible! I'm currently building a new coop and run as laws have change here and I can have 10 hens (legally)...shipping on 10 chicks though was $50, by ordering 5 more the shipping dropped to $15! I "saved" $25 by ordering 15 chicks...this is how chicken math works though...building a run onto our unused shed (it came with the house...we've never really used it for anything and almost everything in it "came with the house"), at minimum, the run will be 7'x30', but will be wider than 7'...I just have to "wait and see" what kind of lumber I have to work with when my mom and stepfather arrive next week! They are cleaning up their property and bringing me all of the lumber (1"x1" to 2"x12" and plywood).
TL

R....build bigger and plan extra space so that they have plenty of room to stretch out.