It is 1609 meters in length1 and is, according to the Admiral Ackbar, heavily armed and armored. However, his comment also seems to indicate that ISD has a greater percentage of shorter-range cannons as compared to Rebel cruisers, thus giving it advantage at shorter range.
Like many other capital ships in Star Wars, it combines roles of battleship and aircraft carrier. It's heaviest weapons are, like its predecessor , located in double turrets at each side of bridge superstructure.
Imperial Star Destroyer is called Imperial class or Imperator class in fanon.