Just finished reading Tarkin. Although it has no major bearing on the story, for a brief moment it talks about a secutor class Star destroyer. 2200m long, is canon. Twice the size of an ISD. There's our extra large ship, folks. Playable on our board size and all. It's a carrier, and about the armament of a normal ISD. It carries twelve squadrons. I'm not saying give it that many squadron points, but it could be a fun carrier. On the opposite end of the spectrum, is the allegiance class battle cruiser. No squadron compliment but twice the firepower of an ISD. 2200m. Not sure if this one is canon though.
ISDs are 1600m long.