Fortunely, HERO rules doesn't cover size specifics, but rather the capabilities of a vehicle. Size will very dependent of the kind of technology available. Early 20th Century flying machines would include airplanes, rather small but with a reasonable cargo, and zeppelins, very big and a with a relatively small cargo. In classic Star Trek, only very big ships could travel between stars, whereas in Star Wars FTL fighters were common. Do whatever seems reasonable in your setting.