10 sq ft for pen is the guide or target to prevent antisocial behavior in the flock. I've got a 20x30 pen for 25 hens and haven't seen any stress with a 4x8 - small coop. They are allowed outside to the run everyday. Plenty of roost space and they cram into the corners anyway. Apparently, they like the window seats.
All this is a general rule and there are exceptions to every rule (except this one). Lots of variables and temperament of your flock members being key. Or so I'm led to believe. This is my first flock, so I'm reading and trying and having a blast. Take all internet advice with a lot of salt.
Best advice I've been given:
Try it.
Start small and grow as your interest and experience and comfort do.
Listen to your birds and watch for behavioral clues.
Small is a relative word.
So what breeds and purpose are you considering?
I wanted meat and eggs (dual purpose = DP). Wife demanded leghorns (eggs) and I ordered Barred Rocks(DP), Rhode Island Reds(DP) and got some Production Reds(eggs). The leghorns are skittish and aggressive. The Barred Rocks are the most docile with the RIR in between. From research, the RIR were supposed to be the meanest and most aggressive ones. Now, mine are not mature yet, so things may change with the onset of egg laying. Mine have been handled almost daily and all are tame (they come up to me and I can pick most up with a little fussing). Compared to my brother's flock where he couldn't get within 5 feet of his hens with a handful of food.
Best wishes!