Depends mainly on whether you have machinery to move it. I mean, 10x20 *buildings* can be built on skids, and pulled from place to place by a team of horses or a large (actual) tractor.
If you mean something to move by hand, though, 10x12 is about the biggest anyone does and still have it at all moveable, and what you get at that size is really only suited well to broilers and/or summer grow-out pens, not year-round accommodations of layers in a climate that gets winter; also what you get at that size is not very predatorproof at ALL, so you need to have the whole tractor in a pretty seriously predator-secure area (which few people have) or be prepared to lose batches of chickens sometimes.
If you mean year-round housing for layers in a climate that experiences wintertime, it is real real hard to build something for more than 4-6 birds *maximum* and even with that it'll be hard to move and they won't have lots of space.
GOod luck, have fun,
Pat