If you want the birds to be able to move around and eat grasses, your pen will ned to be fairly large. If you just want them to stand there, then 10 sq ft per bird might work. One of the things I like most about having turkeys is watching their behavior, which requires room to move and other things to eat besides feed out of a bag. But that's just me. I just built a new breeding pen for two or three turkeys. Its not quite square so I haven't figure the area, but the perimeter is 170 ft.
To keep out critters, me fence extends out on the ground about 18 inches, where there are two strands of electric fence. Nothing inside so far, but I have found evidence of three attempts.
My turkeys don't go inside after six weeks. They roost high, about 8 or 9 ft up, rain or shine. The breeders stay outside in the snow and seek shelter in the trees when it gets too cold. They fly out fairly often in the morning, but they usually want to go back in.