I picked up a 1939 Billings and Spencer catalog when I was at Donnelly's last week. It shows as the largest Engineers wrench a whopping 4 5/8 in. opening. Wrench is 42 in. long and has a head thickness of 1 7/8 in. Unfortunately they don't list a weight.
Wrench is made from carbon steel, and cost $58.50 in a plain finish. If you wanted the fancy version, that would be the one with the black crackle finish and the polished head, then you had to pony up an additional $3.50.
I would guess that there are probably even larger ones, maybe from Williams.
Mike