The Severn dredger (http://www.vesseltracker.com/en/Ships/Wd-Severn-7430931.html?show=photos) (IMO 7430931) seems to be 230ft long and has a beam of 46 ft.
But there are indeed dredgers of all kinds and sizes, as many as there are waterways having various bottoms, sand, gravel, mud, cars, anything.
Did you already know how much materials "Iron Flounder" will load? It is indeed at this stage of the design that you can best check the underwater volumes: unloaded, with the propellers just near to the surface, and loaded at maximum draught, water almost at the main deck. It is of course that volume that will define the kind of dredging work, and I am convinced there are more small dredgers than big ones.
Regards,
Luc