Well this is BIG!

[CINDERS]

The Biggest naval cannons of WW2 was the 18.1-inch (46 cm) naval guns equipped on the Japanese battleships Yamato and Musashi.

Dora railway gun - HOLD MY BEER!

A massive 31 inch/800 mm shell from the German 'Dora' railway gun next to a Jeep.... shells weighing 7 tons and had a range of 29 miles/47 km!

The jeep adds a good context to the actual size of the projectile which I think I've seen a projectiles pic of one at Bovington.
Not sure if these weapons lead the USA to try the HARP cannons.

Schwerer Gustav Was the Largest Gun Ever Made -