I like reading about canon characters primarily, with OCs filling out the universe.
I don't have a preferred length; however long it takes to tell the story well is long enough. When a story starts to ramble, I ditch it. I can usually tell when an author is making the story up as he or she goes along, rather than having a completely thought out and written story before it's posted. I don't have patience for that, not only because the story isn't tight, but also because literary devices like foreshadowing are non-existent.
I typically don't like romance (there are some exceptions) because I'm not a fan of soap opera in space. I don't 'ship, ever.