I would recommend a walk in. If you are using pallets, you could easily expand to 4 x 8, and just tie 2 pallets together. If I were building a pallet coop, I think I'd try to build it with a soil floor. That shouldn't be too difficult if you use T posts to anchor the pallets to the ground for your framing. Then you could attach your skirt to the outside of the building before putting on your sheathing, or what ever you use for your walls.