Just a suggestion... may not be worth anything, but with all the size possiblities already in the game for ships (not discussing bases) maybe the standard should not be so much size but installable features? For example a higher allowance of engines, more armor slots, etc.
Installable number of engines is purely a mod-based feature, not really something that has to do with models. Armor slots, maybe, but your mod can simply use a slot override file if you need to do something fancy and unusual.