I have a mix of Welsh Harlequins, a Cayuga, and a mallard. The mallard started laying about a month ago, and her eggs are pretty big. I'd say they're bigger than jumbo chicken eggs. The Cayuga's eggs vary between XL and jumbo.
But I've been disappointed with the Welsh Harlequin eggs: I don't have a scale, but I'd guess that they're only large at best. The birds are about 8 months old and have only just begun laying. I thought duck eggs were supposed to be bigger than chicken eggs, and that's what I put in my Craigslist ad. Can somebody offer me some advice? The ducks are free-range during the day and cooped at night. I feed them laying pellets and they have plenty of water to play in. Does egg size increase under certain conditions? I hope so.