The Eclipse-class star dreadnought are an extremely large warship, of which only two were ever made. Both were constructed to serve Darth Sidious's personal flagship. They're 17 kilometers long, a bit shorter then the Executor-class shipss, but were much wider. Considering the initial statements about the size of the Executors, it's fairly obvious the Eclipse ships were meant to dwarf them.
Eclipses were among the most powerful warships to appear in Star Wars. They had enough firepower to engage entire Rebel fleets at a time, and their armor and shields were so strong they wouldn't need to think twice before ramming other ships (one these rammed through the Galaxy Gun without taking any damage). In addition to their standard weapons and huge number of fighters, they also possessed gravity well projectors to keep enemies from escaping inter hyperspace. The crown jewel of their features, however, was their own superlaser. Though not in the same league as the super-lasers possessed by the Death Star, they were still powerful sear of continents and/or crack open a planet's crust. The resulting heat and superheated chunks of the planet's crust being ejected would likely kill everything on it.