A capital ship was any armed military starship over one hundred meters long. Ships of this class ranged from small and nimble corvettes like the Corellian Engineering Corporation CR90 corvette to titanic Star Dreadnoughts, such as the Kuat Drive Yards Eclipse.
Why do capital ships in Star Wars appear to fall in space, where no gravity should exist? The real answer is because it looks cooler. Star Wars has always followed the Rule of Cool over any real laws of physics. That’s not to say physics never apply, but it is why we hear sound or see characters survive in vacuum with very little protection. Irvin Kershner said he considers Star Wars a fairy tale and not science fiction specifically because he doesn’t have to worry about science, and he appreciated the freedom to be able to do anything he wanted. And remember, Star Wars was based off of old serials like Flash Gordon that were created in 1938 when our understanding of space wasn’t nearly what it is now.