Mabye SASD? Shivan Advanced Super Destroyer?
SFS? (Shivan Flagship)
Realisticly though, lets look as some ships:
Name/length (approx)/class
GTVA Collossus (sp)/6k/ Superdestroyer.
SJ Sath/5k/Juggernaut
GTCa Warlock/4.8k/supercarrier
GTD Hecate/2-3k/Destroyer
EASD Nemesis/5k/Superdestroyer
SPD Vindeshack (sp)/some obscene length/Planet destroyer.
So as we can see from this hastily-constructed table, the major difference is between Destroyer and Super-Destroyer. The only Difference IMHO between a SuperDest and a Juggernaut is Firepower and Armour, not size.
So depending on Armour, weapons, and importance in the campaign (Is is like a Sath? like a pawn that gets flown out every mission by the Shivans?) Because IMHO SuperDestroyers are more suited to the One-of-a-kind-blow-ya-head-off kind of ship as opposed to the Big-but-cheaply-mass-produced-in-china-but-still-blows-ya-head-off-though-not-as-well-as-the-other-kind-of-ship.