Honestly, I think that it's likely that Executor and Lusankya were the only units of their class. The basic Imperial doctrine is one of terror, and if you have defenses that can see off a star destroyer or two, the level of terror won't be very different if you see an 8 km ship or a 19 km one. The Executor was built to intimidate the really heavily defended planets, like Kuat. The Lusankya actually clawed through Coruscant's planetary shield. I imagine this couldn't be done at Hoth because of the ion cannon. Torpedo spheres were probably cheaper, and used for less important planets. The imps may go for overkill, but there is such a thing as too much cost.
The canon is deliciously ambiguous on this point. While Executor and Lusankya may have been the only two of their class, I think that ships in the 8km and up size range were fairly common in the Imperial Navy. I also strongly disagree that there's not a big difference between an 8 and 19 km long ship. The level of terror will be very different to any opposing admiral who knows their job.
Assume Saxton's idea that an energy armed warship is one that can direct most of the power from it's reactors through its weapons at a target, then the effectiveness of such a warship can be estimated to scale with that ship's available power. A reactor's power output is more closely estimated by looking at volume vice its length. Assuming that the reactor dimensions scale the same way as the ship (which actually isn't a bad guess), the difference between an 8km ship and a 19km ship is:
(19/8)^3 = 13.4
All else being equal (it won't be but this is order of magnitude stuff) this means that a single 19km ship could defeat 13 of the smaller 8 km ships. Yes, I know that this is not the way that modern warships work. That's true - but we don't use directed energy weapons on modern naval vessels. The size of a Earth wet-navy ship's powerplant right now has no direct relationship to its destructive potential. In Star Wars, you can make the case that there is a very direct relationship.
Looking at the difference between Executor and an ISD is even more interesting. The difference between the 19 km ship and a 1.6 km long ISD is:
(19/1.6)^3 = 1674.6
So, this is the origin of Saxton's comment in the RotS ICS book that a single Mandator-II class could take on 1,000 Recusant-Class destroyers
In the end, I think that practical considerations will bring that number down to a much lower figure (there were at most hundreds or tens of Rebel Mon-Cal at Endor after all). And I suspect that the limits have to do with shielding progressively larger and larger ships getting less effective past a certain size. But I still think that limiting the number of bigger ships takes something of the wonder out of the GFFA. To use a Tolkien analogy, I don't think of most star dreadnoughts or battlecruisers as Balrog level...they're more like Dragons and Oliphants. Executor maybe is one of the Ring Wraiths just because it is a ship designed to not just win fleet actions, but subjugate pretty high end star systems.
The Eclipse...The Death Star...now those are Balrogs...or Sauron himself.
Edited by feld, 17 December 2009 - 04:04 PM.