Depends on the duck size. From the title, you're talking Muscovy, so they are probably the largest domestic breed of duck and would need the most room.
I remember reading that Majestic Waterfowl recommends 20x20 pen per duck - they have Pekins and Muscovies mostly, I think. For a night shelter, I think Storey's Guide to Raising Ducks says 6 square feet per bird minimum...I need to check on that.
And Storey's Guide also recommends that if the ducks don't have access to additional pasture, to go with a minimum of 25 square feet per duck.
We have Runners, less than half the size of Muscovies, and they have 100 square feet for 13 ducks, but that is just their day pen. They have supervised access to a quarter acre of woods and gardens twice a day. And their night pen is also about 100 square feet.