Colony-scale planning begins with the physical size of the transport. A crew cannot remain healthy for nine months in a space no larger than a small shed, especially when the mission continues after arrival. Fitness facilities are necessary to limit muscle degradation during the voyage and prepare travelers for gravity changes on Mars and on their eventual return to Earth. Teams building permanent infrastructure may need two or three years of continuity on the planet, turning a round trip into roughly four years away. The ship must therefore provide long-duration living and medical support space beyond what a launch capsule offers.
Chapter · 36:37-39:08
A Mars transport must support years away from Earth
Ship size follows from habitable volume, fitness facilities, gravity adaptation, and the fact that infrastructure teams may spend several years away rather than make a short visit.