There's a little of both to tell you the truth, and there are many types of weapons used. Some quite simple and very much based in reality (Railguns), and others not so much (Meson weapons).StarSword wrote:For combat vessels another consideration is what type of weapons your 'verse relies on. For example, ships in Mass Effect tend to be very long, specifically, because that provides more room for the spinal mass accelerator that runs most of the length of the ship. This allows them to fire the slug with greater force. Because of this, dreadnoughts are usually around a klick in length and (according to the codex, anyway; the video artists weren't paying attention to this) act like artillery pieces.
By contrast, a setting that mainly uses omni-directional energy weapons (e.g. Star Trek, with the phasers on a GCS organized into strips that can theoretically emit a beam from any point along their length) have more freedom in design.
The other consideration? Psychology. The Galactic Empire builds huge ships partly because they look scary.
And as far as big ships to look scary go. War ships in this setting don't have windows and the bridges are usually buried under several decks towards the core of the ship, this has as much to do with the way artificial gravity works as is does with aesthetics.