The Gen1 USS Enterprise is a huge ship. At .6 miles in length it’s bigger than any craft or building ever constructed. It’s nearly three times as long as the largest US aircraft carrier, and its length is greater than the tallest building in the world.
Why is it so big? First, it must house a gravity wheel that is large enough in diameter so that people are comfortable inside of it and the behavior of the artificial gravity to them seems reasonably Earth-like. Second, the Enterprise is a combination of spaceship, space station, and spaceport. This means it must support having many people on board at once – up to a thousand at any given time. It must be able to dock and refuel multiple smaller spacecraft at the same time. And the huge cargo-carrying capacity is critical for hauling probes, landers, and base-building equipment to Mars and elsewhere. Simply put, if we want to get serious about establishing a permanent human presence in space, with robust and sustainable capabilities to do big things up there, we need a big ship.