The largest practical gun for a wartime destroyer

What was the largest practical gun that could be mounted on a Second World War destroyer hull?

Although very large guns might fit, practical armament stayed around 4.5 to 5 inches because rate of fire mattered more at destroyer fighting ranges.

Physical installation is not the same as an effective fighting system. A very large late-war destroyer might physically accept a six-inch weapon and perhaps even an eight-inch gun. Recoil, stabilization, ammunition handling, and the hull's motion would make that theoretical installation difficult to use. Destroyer engagements rewarded rapid repeated hits more than the largest possible shell. The practical choice therefore stayed around 4.5 to 5 inches, especially in twin mounts with a useful rate of fire.