In the Dutch coastal town of IJmuiden, the Germans built two gargantuan bunkers for their Schnellboots (called E-Boats by the Allies). Only one of them survived the war. That bunker is 247 meters long, 74 meters wide, 18 meters tall, they do not get much bigger than this!
In September 1944 the Germans gave up on finishing the construction of this bunker, insistent allied bombing and a change in the fortunes of war caused them to stop.
Note: A grand slam was not used on this bunker, it was had to endure multiple attacks with the Tallboy.