Yes, of course they are big. Everyone knows they are big. That’s their thing.
But in the movies we only ever see one of them next to a Corellian Corvette, which is one of the very smallest ships that is still considered in the capital ship category. A number of additional capital ships have become well established in the Expanded Universe over the decades, each with their own listed lengths. But seeing the numbers on paper usually does not really give a true sense of the relative sizes. A ship that is twice as long as another ship of the same shape is not twice as big in total mass and volume, but eight times as big.
To better visualize this I took images of the various ships that make the most frequent appearances and put them together side by side at the same scale. And it turns out, Star Destroyers are really big.
In the movies, the Imperial Star Destroyers are simply very big, with no real reference to how they compare to other large warships. But against the most common ship types of the Expanded Universe, they are still absolutely massive. There really isn’t anything in their weight class except for the occasional obscure one-off appearances.
Because the Mon Calamari MC80 cruisers are over a thousand meters long, I always had assumed that they are comparable to Imperial Star Destroyers. But seeing them side by side that really is not the case. The MC80 is actually most comparable to the Victory Star Destroyer, which is usually seen as the miniature version of the Imperial Star Destroyer.
And again, the Victory Star Destroyer is not a small ship itself. It is still absolutely enormous. When Dreadnoughts were introduced in the Expanded Universe, they were usually portrayed in a way that made them seem like extremely large and powerful ships, or at the very least would have been during the Clone Wars. Apparently the biggest ships the Old Republic had in its fleet. But at 600 meters in length and with a relatively narrow shape, they are already dwarfed by a Victory Star Destroyer and appear almost tiny next to an Imperial Star Destroyer.
When I create stuff for Star Wars, I always try to take the three movies at face value, taking them as my reference frame for what the Star Wars universe is by default. But in light of this comparison, sending four Imperial Star Destroyers into an asteroid field to chase the Millennium Falcon was absolute overkill. And as much as it pains me as someone who regards The Empire Strikes Back as the best movie ever made, the Super Star Destroyer was just stupid.
When it comes to having Imperial ships make appearances in Star Wars adventures, I think Star Destroyers should be reserved for scenes of particular significance. Using them as the go to Imperial warship for most common space encounters lessens the impact their incredible size and power can have. The appearance of Star Destroyers near a planet where the heroes are on a mission, even a Victory Destroyer, can be used as a very effective signal that the stakes have been raised well above normal. Because when there’s a Star Destroyer, there’s always a huge number of TIE Fighters, and even larger numbers of Stormtroopers that could be on the ground. A Star Destroyer is a threat you can not really fight. Your only options are to try to hide or to run. But unless you have a huge fleet on your side, you’re not going to defeat it in battle. And if you manage to get on board of one to sabotage it, your only hope is to evade the thousands of Stormtroopers. You’re not going to defeat them and capture the ship.
It’s just too damn big!