Why modernising a wartime hull starts at cruiser size

What is the smallest Second World War or Korean War hull that could be brought back for modern warfare?

Probably a cruiser, because the full set of modern combat systems and power generation needs a hull of many thousands of tons.

A cruiser-sized hull is the tentative lower limit proposed for returning a Second World War or Korean War ship to contemporary warfare. Modern missiles and radar require space, as do the computers that support them. Power generation adds another demand on the hull. The comparison is with modern escorts displacing many thousands of tons, so it is a broad size judgment rather than a completed conversion design.