GENERATION SHIPS: To Start Life at The Nearest Star System, This Is How Big a Spaceship We Would Need. “Interestingly, even if we double the length of the spaceship, we find a structure that is still smaller than the tallest building in the world – Burj Khalifa (828 m; 2716.5 ft).”

I should note that an Orion nuclear spacecraft that size is quite practical, and could achieve velocities sufficient to limit the travel time well below the design parameters used here.