We have a 5 x 5' pen for our 10 meaties. With that size, I could have up to 16 meat birds, but I like to leave the extra room for their water and feed. They free range all day (if they choose to come out) and I've found they are a lot more active than I thought they would be by allowing them to roam around. No lazy meaties here at four 1/2 weeks (So far...could change shortly). Its great to see them run (well, waddle-run) around and chase each other.
1.5 square feet per bird is recommended for the pen, but if you have room to fence an outdoor area for them to roam, or can just let them free range (with access to their pen/coop) I think you get even happier meat birds. I believe it can take a little longer for them to get to weight, since they are eating bugs and grass, not just chicken feed, so that is something to consider if you are in a hurry to get them culled at 7 weeks.
We put straw (a small amount and I have been told that mice can be a problem so consider that if you use straw) in the pen for bedding and move the pen every few days. Once moved, we rake the old straw, let it dry in the sun( sun? sun? what is that?) and then move it to the compost bin. Hose down the area to get rid of any remaining poo. Even with them free ranging from 7 am to 7 pm, there is a massive amount of poo in the pen, so it is really important to move your tractor/pen as often as you possibly can. Many people on here move it once a day.
-Jessa