Everyone's process is different, but I'd say tell the story you want to tell, regardless of length. Once it's done, then you can look for extraneous (or necessary) narratives, characters, plots, etc. I've always found it FAR easier to cut than to add, so I tend to let my stories stretch a little (before crushing the life from them in the editorial process).
Having said that, that approach certainly won't work for everyone. I know a lot of authors who start off with a solid outline or an end in mind. On the flip side, I know a lot who just start writing and see what pops out. The trick is, as in everything about writing, finding the strategy that works for you. Not great advice, I know, but speaking from experience, no amount of advice, seminars, panels, workshops, and discussion can wholly make up for the amount of trial-and-error it's going to take to find the best way for YOU to write.