Honestly, I've never seen my muscovies use their shelter. I have three birds and the shelter is about 2x6. It has an opening at one end (OK, so it is built kind of like a desk only with solid sides, top and front, and the bottom and back are totally open, but I pushed it against the coop so there is just enough room to go in the one end--the chickens think it's great), and is layered with straw, but they never, ever, sleep in it.
To be honest, your birds won't use it much anyway, all you need is enough room for them to get out of the weather if they want to...Now, if you plan to keep them shut in there during really awful days (and you expect a lot of those), maybe you'll want a 4x8, but unless you plan to make your flock much bigger, you probably don't need half that much space. I guess it also depends on whether you have a secure run for them too, or just the shelter. Mine sleep in the chicken run because my geese pick on them.