Ducks don't coup, they sleep on the ground. I have a mixed flock, chicks and pekins, my ducks sleep under the hen house (and sometimes, under the 275 gal water tote). Anything raised more than a few inches is no good for them.
Not sure what your climate is, or how long you might expect to keep your ducks couped up during really horrific weather, but in general the run should be bigger than the coup, and the coup needs for ducks are significant. I have about 110 sq ft under roof for my ducks - its more than I need for my 7, and should be adequate for 12 (assuming all 5 eggs in the incubator make it to hatch day), but my run (shared with chickens) is over 1,000 sq ft, and my birds free range all day. (It took them no time at all to decimate every green thing inside the run).
You can do with less, but the smaller you get, the more likely behavior problems will begin to crop up.
That said, I do not have years of experience. Maybe someone who has been doing this for decades can hop in and provide some less conservative advice than my own.