I don't know how run space is calculated, but I figure it's big enough if the grass can keep up with the population of chickens foraging and scratching. If they turn it into a barren wasteland... Too small!
General rule of thumb is 4 sqft per bird in coop, 10 sqft of run per bird (you can do less, I have used anywhere from 8-10 sqft total for birds depending on their size). Even at those first larger numbers they're still turn it barren. It's been a while since I looked into it but I want to say for them to not destroy it the requirement is something like 200 sqft of pasture?
Edit: I currently have three rescued hens in a 16x24 area, which works out to 128 sqft per bird and it is barren dirt. Just as an FYI
Hi Matt.... not wanting to be argumentative, just discussing....
I've more often seen/heard 3 sq. ft. per bird for the coop. And I would agree and even go a bit less if necessary based on my experience because....
my birds are inside only to lay and sleep. They
never go inside otherwise, so they are never milling around on the floor. My nest box is even taking up some of that sq. footage.
My pen is covered so there is no need for them to go inside during the rain or snow.
Also, we should mention for newbies, the 10 sq ft per hen outdoors is the
minimum recommended, just to keep them from pecking each other, etc. And coop space doesn't substitute for run space. (Suggestion.... lift the coop off the ground 3' and let the girls use that ground space.)