A friend from Facebook admitted the other day that she thinks in novels. By that I think she meant that story ideas come with big, involved plots that require novels to resolve. I have the same affliction. Much of my fantasy world is involved with very involved plots and would take many novels to describe and resolve. Hell, the Aglaril Cycle, my current series has a very simple premise but to do it justice, I'm estimating eight novels: one to find each gem and then one to wrap it up.
But not all story ideas require novels, obviously and I have been trying to think in short stories. That's a lot harder. A good short story has to be self-contained. If it is not, it has a novel-feel to it. I have written stories that I thought were complete but the readers didn't think so. They wanted more, expected more. And I have written stories that are complete. The trick is to determine if all the questions raised by the story are answered.
For example, a baby is found in a ruined tower. How did the baby get there? Once that question is answered, the story is complete.
Example 2: A brave -- or foolish -- warrior seeks out the oldest dragon in the world for battle. What happens? Does he win because the dragon is so old? Or has he underestimated the beast's power?
Example 3: A struggling writer discovers a magic pen. Does it help him achieve his dreams of success or force him to write things he would never consider putting his name to otherwise?
How you answer these questions will determine the story you get. For example, my answer to the first question was the baby is the regression form of the alchemist that had lived in the tower. Facing death, she experimented with a potion that would place her in stasis and heal her wounds. But the healing also aged her in reverse so when she is finally found, she is a baby. To make the story more interest I also decided that the potion had no effect on her mental abilities. So she is an adult woman trapped in the body of a baby and she's not a nice person.
You can see how I embellish the idea a little. Then I let it take its course and write down the results, which in this case was a 4500 word story entitled Baby Muran.
I did similar things with the other examples.
The more questions a story has, the longer it is. Likewise, if you start adding questions about the characters you end up with a novel. Short stories don't have time or space for character-specific questions.
So regardless of what you are writing, try to determine the questions you need to answer and make sure you answer them because satisfying the reader is job 1 when writing fiction.
Keep writing everyone.