Justin:
The plans I have don't show a rated capacity. I've never felt that the boat was overloaded weight-wise with myself, a large dog, motor, gas can, decoys, etc. Keep in mind that I tend to use the Widgeon in more sheltered, remote places---small lakes and sloughs. I have a 16 foot Devlin Black Brant 1 I built many years ago (yes, that dates me) that I use as a big water boat.
As Dave mentioned, the real limitation is the size of the cockpit. I lengthened the cockpit when I built the boat versus what the plans specified. I generally sit on a low, folding beach chair with my dog lying down between my legs. The chair keeps my butt an inch or so off the floor and allows my knees to be slightly bent when sitting. You don't have to be a contortionist but it's not spacious. I haven't yet put the boats away for the winter so I could take some pictures if there are some particular shots you might be interested in.
Craig:
When I think of layout hunting, I'm thinking of open water hunting a hundred or more yards from shore. I have a one-man Busick design layout boat that I built between the Black Brant 1 and the Widgeon that works great as a layout boat. Using it necessitates a tenderboat (and a second hunter) since the layout boat isn't very manueverable. Additionally, there isn't any place for a dog. I used to have a Kara style boat that worked reasonably well for layout hunting. You could probably use the Widgeon as a layout boat if you built it with a removable motor mount. I would feel comfortable lying in it in some pretty rough water but am not sure that there's room for a big dog as well.
Again, I can post some additional pictures if there's something particular you'd like to see. Also, I'm not sure how far you live from the Okoboji area but you are welcome to take a look at my boat if you are up to the trip.