Chicken Ark size

I was contemplating building a chicken ark to house meat birds for a couple months at a time. I don't want it to be too big and I'd like to be able to move it alone. I was thinking something 3 to 4 feet wide and 5 to feet long, maybe up to 8 feet long. So that would be anywhere from 15 to 32 sq ft for birds. Now I've read that cooped chickens and ducks need anywhere from 3 to 5 square feet each but in practice how many birds could I keep in that size ark for only 2 months at a time? Or maybe someone could tell me what a good size ark would be for 12 to 15 birds without cramming them in there. I mean I'd like to be as humane as possible raising my own meat birds so having them in commercial conditions would kind of defeat the purpose.

 

Thanks forthe input. I think I do like 4 x 8 as dimensions for an ark. I may just build one that large then let them free range when they looked cramped and get big enough. Then the ark could be their night coop. I have yard space for birds to free range since I already have free range birds but I was thinking of using an ark so they might fatten up more and have protection from hawks while still young.