I've always liked a good short, something that captures the attention, gets me involved, keeps me involved, and provides a satisfying ending, all in less than an hour. I like to immerse myself in the story, and I like to read a story in one sitting; so sometimes I just can't pick up a full length book, especially if I know I won't want to put it down.
It makes sense then that I have a fascination with writing shorts as well. I have a couple of books started, but I keep getting distracted by shorts. So, how do I know when I'm writing a book or a short?
Books are easy. I know it will be a full length book if mapping out the basic ideas results in +25,000 words.
Shorts are easy if...
The scene that is the basic idea is short. (I say scene because all of my ideas start from a scene that repeats over and over in my head until I write it down.)
What had to happen for that scene to occur is short. (I can't hop straight into an action scene.)
The consequence of that scene resolves itself quickly. (You can't have too many consequences lest the story become something other than short.)
I think the most important question is how do I keep the shorts short?
I don't always. I've had one short that just refuses to be a short. The characters demand to be heard, and I've fallen in love with them and their story; so I've decided to keep writing until we are all satisfied. I'm anticipating that it will be at least a novella, or maybe the beginning of a series.
As for the rest...
I start by keeping the basic story between 3,000 and 5,000 words. That means bare bones. Then, I polish. I add the descriptions necessary for interest, for flow, for speed.
I don't change the basic premise. I don't say but what if I add this twist and that twist and ooh add this character and explore how the whole story affects him/her in addition to the main characters (that's a whole other short or book if you want it to be). You get the picture.
Hope this helps!