The largest artillery pieces ever made were the Little David mortar made by the United States in 1945 and Mallet’s Mortar made by Britain in 1857. Both had a caliber of 914 mm, which is about three feet. Neither were ever used in combat.
Little David mortar William Lawrence Sr Omnibit 8-7-18
Largest *by calibre*. They weren’t the largest pieces, in terms of sheer size. Railway guns were far larger, as are naval rifles-the guns that make up a battleship’s main bettery-but they weren’t throwing a 914mm shell.