Funny you should mention B5.  I loved it myself, still do... but one of the first things that comes to mind, in this context, is that while JMS got a lot of stuff right, it was quickly pointed out to him that the *air alone* inside the station would mass more than the first number quoted.  Which he, of course, pulled out of... *ahem*.

If you want to play fantasy, fine.  The numbers tend to be smaller, more manageable.  Little ships, little kingdoms, on the surface of a single world.  When you get into science fiction, all the numbers get bigger. Much, much bigger.