I'd make it a little bigger, especially if you have large fowl. Cocks can get pretty tall, and they will try to squeeze in more than one at a time. I think mine is 12"W X 18" tall, and I had one cock who had to duck his head.
Here's the door we did. Right now there's a board on the inside, screwed to the wall covering the door area. Until we get the outside done. I can't wait to see how excited the girls get when they come out for the first time.