Write Away

This is exactly how I see it. The length of time that passes is a big factor - a story spanning months with action happening throughout is more than likely not going to be short, unless it's perhaps a series of vignettes. For the story itself, the number of conflicts and steps involved in each is a big indicator.

That being said, my default mode is novel and I look at any idea in terms of how much I can expand it and deepen it with no ability to do the reverse.