One Inch is 2.5 cm, so a 12" x 12" is 30 cm x 30 cm. For individual nesting boxes, I'd consider 30 cm x 30 cm the minimum. However, here is a post by Digits from another thread that might help you. I think he gave credit to Robert Plamondon for it.
"The traditional community nest is a box four feet wide and two feet deep, with a doorway thats not much more than six inches wide and eight inches tall (and maybe with a flap of cloth or plastic across it). This is good for fifty hens. Another variant is the tunnel nest, which is eight feet long, two feet wide, and has a small entrance at each end. This is good for a hundred hens."
You might want to do a search for more specifics. You do need to hinge it so you can gather eggs from the entire community nest, probably hinging the roof instead of the front or back.
