It'd have to be pretty darned big, to sustain a population large enough to remain viable for a couple of centuries. You'd then have to figure out how to get enough delta-v on it to escape the solar system, but then you'd also need a way to get yet more delta-v at the other end, to slow enough to get captured in a useful orbit, or else fly right on out the other side. Assuming there's a planet you're aiming for, you'd want to establish an orbit of that. So this has to be a small asteroid scale ship, with propulsion that works, after centuries of micro meteorites and radiation, and possibly substandard maintenance.