They do seem quite big, and some Rouens I guess have a line and a dash like these guys, although some threads on here say TSC sometimes stocks mixes so literally who knows haha! https://www.feathersite.com/Poultry/Ducks/Rouens/RouenDLing1.JPEG
Feedstores aren't always the best at identifying breeds (and honestly, ducks are harder than chickens in my opinion) seeing them fully grown will be the true tell : D