IIRC, gauge is a function of bore diameter and weight of shot. 20 gauge means that it takes 20 lead balls that diameter to weigh 1 pound. Ditto 16 balls to the pound for 16 gauge, 12 balls to the pound for 12 gauge. So in a theoretical 1 gauge shotgun, the bore would be such that 1 lead ball would weigh 1 pound. So a .12 gauge would be, as somebody has already stated, the size of a field artillery piece or maybe a 5 inch dck gun.
Or, on the other hand, I guess the .410 gauge measurement standard could be used, .410" being the actual bore of the shotgun. In that case, a .12 would be about 2/3 the size of a BB (.177 cal.).
Now I'm gonna pour a shot of Chivas and ponder what I just wrote...
12 gauge is approximately .50 caliber