If you have broad-breasted birds and just want to raise them for meat, you don't need much room at all. Most outdoor dog kennels would be plenty big. If your raising heritage birds and want to give them some room to stretch their wings (literally), your pen could be as small as 20 by 30 feet or as large as 100 by 100 feet or bigger. OK, 100 by 100 is pretty big for 3 birds. My main pen is about 120 by 90 and holds about 25 birds with lots of room to spare. My secondary pen is about 60 by 30 and only has two hens in it right now.
So the answer is it depends what you want to do with your birds.
Oh yeah, I don't have coops for mine, they mostly just hang out outside. They have trees for shade and some roosts to sleep on. I have six foot fences with no netting. I have to put a couple birds back everyday. I don't like ot clip wings because I have witnessed some bad crash landings into hard objects. If they get out, its very easy to shoo them back in.