Early DesignsThe Acclamator assault ship was the early design that led to Star Destroyers. The Clone Wars really kicked the Galaxy's economy into high gear, and the war machine started. These starships could carry a ton of troops and vehicles and drop them right in the middle of a battle.
The Venator class was the first ship called a Star Destroyer, and began service during the Clone Wars. These were still in use for many years after the Clone Wars ended, but were not as popular after a couple decades as the newer designs.
The EmpireThe most common Star Destroyer, the Imperial class, was big and intimidating. Tons of guns and starfighters, along with more troops than you could count. Simply design, looks like it's gonna stab you. And if it does, it will definitely hurt. It was so big, it could reverse birth a blockade runner which is fascinating. Still, they got bigger. Keep reading. You'll see.
Super Star DestroyerDarth Vader was given the biggest Star Destroyer, the Executor, as his personal flagship. He used this ship to lead a small squadron and hunt down the Rebels, specifically Luke Skywalker. Vader initially requested that a couple of nifty racing stripes be added to the ship just to break up the blah gray thing the Imperial Navy had going on. The maintenance crew told Vader that by the time they finished painting the racing stripe at fore of the ship, it would be time to start repainting the stripe at the aft of the ship. Vader relented while committing zero force chokings. It was an early sign of the good in him.
In case you can't appreciate the size of this damned thing, here's a little more anecdotal evidence. There was a 50,000 watt AM radio station broadcasting at the aft of the ship. The troops at the fore could not get good reception. Vader had 14 state rooms in the massive ship. Each one was a four hour walk apart. If you picked a banana in the hull of the ship, it would be brown by the time your rode the elevator to the bridge. The empire had to install a miniaturized warp drive to expeditiously route time sensitive inter-ship mail. Are these big ship jokes doing anything for you?