Classifying cruisers and battleships in science fiction

How should light cruisers, heavy cruisers, battlecruisers, and battleships be distinguished in science-fiction settings?

Use each setting's relative scale and operational roles, while distinguishing a ship's normal classification from what it can become in wartime.

Light and heavy cruisers in science fiction often differ in relative size or role. The historical distinction by gun calibre arose under particular treaty rules and does not automatically transfer to a fictional navy. Space travel also complicates the distinction between a fast battlecruiser and a battleship. The Star Trek example produces a disagreement over Nebula and Galaxy ships: one view calls them battleships in wartime, while the counterargument treats them as peacetime cruisers whose internal volume and power generation permit wartime adaptation. Similar peacetime capability therefore need not imply the same capacity for conversion.