The Executor-class Super Star Destroyer (also known as a Star Dreadnought) was a class of heavy capital ships built by the Galactic Empire, measuring over 19 kilometers long. By volume, these ships were many times larger than normal star destroyers.
The only flaw known to exist in Executor-class Star Destroyers may actually be a matter of command incompetence, instead. At the Battle of Endor, when a Rebel starfighter crashed into the Executor's bridge tower, the ship went out of control and crashed into the Death Star. It isn't clear why an auxiliary control center wasn't able to steer the ship away from danger in this incident.