None other that Stephen Hawking says the IF the human race is to survive we MUST explore and colonize other planets. The start is to gather materials. Where from? The asteroid belt is chock full of raw material, the exercise to get there and back a training ground for longer trips. As for the size of a ship? We build cruise ships that house thousands of people for weeks at a time. If you work in a zero gravity environment – as in earth orbit – one could build a vessel that could hold thousands of people in an environment that is self sustaining and could conceivable span the vast distance in several generations. But even now there is research on a “warp” drive. So quit being such a tool and expand your mind.