A Planetary Bombardment vesel is a (usually fairly bulbous) ship made for the sole purpose of ruining the enemies on the ground's collective day.
They are only 400m long and 150m wide; with three decks. They are plated with only 2m of Vanadium-steel alloy, have an AI (with the somewhat prerequisite GIGA-level mainframe); and a crew of 20.
Their main weapons are six (three on each side) massive, double-barreled ionic cannons, that, while useless against ships (Because of the nature of the projectiles, they can't cause any damage in a vacuum); will royally screw over anyone unlucky enough to be hit by one on the ground.
The first deck is, as with all Earth Human-made ships, the engineering level, housing a five-gigawatt reactor (to power the lasers), an ion drive system, and most of the mainframe. The second houses the cannons, and all the other machinery required to turn the things. The third multitasks as the bridge, crew quarters, and server room.
To both counteract overpoweredness and to put things into perspective, until ion cannons become easier to produce (which most likely won't hapen in the timescale of this RP), these ships are so prohibitively expensive that most fleets (even large ones belinging to immensly wealthy empires or corporations) will only have two at most.