Does depend on the size/type of sheep you have, and how long you intend to keep them penned.
I have a variety of sized ewes from a small 'Clun' to a big Herdwick.
Herdy had a 6' x 6' at the start but I had to make it bigger as she had twins and I was worried she would lie on them - not that she did, but I'm sure she appreciated the extra room. Shaggy the Clun also had twins but she was okay in a 6' x 4' pen.
As we have foxes galore I let them out during the day for a few hours and penned them at night for a fortnight or until I thought they were big enough to stay out full time.