I've long maintained that the GTVA really doesn't need a warship larger in size than a Superdestroyer. A newer class of superdestroyer, capable of killing standard destroyers, but can be produced in more numbers than, oh, 1.
A type of superdestroyer I was trying to come up with was based off of the Hades-class, but with the tower at a 1/4 of the legnth that it was. The SD would be more cut down in size than the Hades... keep in mind the GTVA would need to accomodate these ships inside a dockyard or a Ganymeade, and would boast long-ranged juggernaunt beams (Same type of overloaded beams that the Collosus used against the Sathanas), two slasher beams, and a lot of AA involving flak and anti-fighter beams. That's more or less my concept in a nutshell. (Although it probably wouldn' work, mainally because we don't really know, aside from the tech room that the GTVA still has the plans or the industrial capability to build a Hades, and at any rate it's be a terribly inefficient warship due to its heavy weapon emplacements. Seriously, they blow.)