Technology folks!

The answer lies in what level tech you have chosen.

Remember when watches, TV and phones where HUGE! Now the only reason they are the size they are is because of our ability to interact with it.

I purpose that with high enough technology the only limiting factor will be .... us! Thats is our size and limited 5 senses. Include a 6th sense (PSI) and it takes on another twist.

So if you have chosen low tech space travel, make engines and support crew large and clumsy,
but if you chose high tech make them small and elegant.

2 cent given