We're building an 8ft by 4ft by 3-3 1/2ft tall(the roof slants from 3 1/2ft down to 3ft) coop for our five ducks. I don't think it really needs to be that big, but oh well lol. It'll sit up on 6inch legs. The door will be able to open up like a drawbridge, so it'll double as a ramp for the ducks to walk up. As my ducks aren't going to get super tall, the doorway will be a foot wide a two feet tall. I agree that for a runner, you might want to make it taller than that.
My neighbor just used a couple of large dog houses and dog runs(to separate their drake from the ducklings). For Texas, that really works well. If you have really cold winters, investing in a heating lamp might be a good idea.