How small can a ship be with triple six-inch turrets?

What is the smallest ship that can reasonably carry a triple six-inch turret?

About 10,000 tons: a triple six-inch mount needs a large, wide, cruiser-sized hull to work properly.

The answer places a practical triple-six-inch-gun ship around 10,000 tons rather than treating the turret as an easily portable weapon fit. The mount needs a hull with enough beam and volume to accommodate its machinery and ammunition arrangements. Stability and structural demands therefore push the design into cruiser scale even if only one turret is contemplated. HMS Belfast provides the recognisable example of the sort of hull dimensions involved.