I must be missing what the purpose of having a star destroyer sized starship would be. Why do the starships need to be so big exactly?
I seem to remember a magazine that said that a Star Destoyer was six miles long thus stil bigger. It has since been described as a mile to a kilometer long. Too me, that seems a bit too small, even if it matches the small M Falcon on the back of the bridge tower.
Large size means deeper runs in space, a large atmosphere, longer legs--greater crew health.