What is the length of a Super Star Destroyer?

Who the hell cares?

The main problem is that there are different sizes for different SSDs, so deciding on one specific length is impossible. Or is it?

I would have to say that 250 launchers is quite sufficent if distributed equally throughout the ship.

 

Honestly, I'd be willing to buy the idea that the Executor and the Lusankya were larger and more heavily armed than most other similarly-shaped command ships. I'd even buy in on the idea that there were 8 km Executor-like (in appearance and design, but not in size) ships such as the Knight Hammer, Reaper, etc. and accept that they're "sisters of the Executor" by means of propaganda (i.e., you build a smaller ship, tell everyone it's the same thing, and get the same psychological effect for half the cost). Regarding the Allegiance, we don't have much of a frame of reference.