I think 'Off-Topic' is probably the most confusing place this thread could have ended up in, given that both Sins AND GalCiv link here...
(Thank god we don't have a dedicated SotS forum, that would end in even more confusion.)
PS: Piznit, cruisers are larger than destroyers.
I think the most common way to map hullsizes to classes would be
tiny -> frigates
small -> destroyers
medium -> cruisers
large -> battleships
huge -> dreadnought
