Are we sure it's large? Because what I got is, "it's large compared to mallards." Everything's large compared to mallards. And yet my Blue Swedish and Rouens fly just fine.Well, being a large domestic breed, it can't fly, which means it can't migrate and also can't get away from predators well, so it's not a good idea to let it try to live with wild ducks. It would be best for it if you can either find where it came from (hopefully it just wandered off from a neighbor's house) or if not, catch it and give it to someone who keeps domestic ducks so it can be properly cared for.
EDT: So, apparently, it can fly. And I'm agreed on finding it a home with other domestic ducks. Sorry if I came off as persnickety.