I found something that shows the bore size of the beast. A 2 gauge is 1.32 inches. That's a big boy.
http://en.wikipedia.org/wiki/Gauge_%28bore_diameter%29
These giant shotguns were generally called "punt guns" and were mounted on small boats called "punts." They were used in the late 19th century to hunt birds. With just one shot you could get a good part of a flock. After big declines in bird populations they were banned from hunting.
They made them even larger with bores up to two inches and even larger.