I was moderator of a panel on how we do short stories at Boskone last weekend, along with fellow workshop members James Patrick Kelly and F. Brett Cox, nearby neighbor Craig Shaw Gardner, and new friend Beth Bernobich. It was quite a fun panel.
Everyone said interesting things, but I most remember the smart thing I said. I don't know how that happened. I'll try to pay more attention next time.
Oh, what I said. One of my questions was how you tell a short story idea from a novel idea. Do have to run some battery of tests on it, or can you pretty much tell from the way it sits in your mind? And what about the idea makes it one or the other? I have to play with the idea for some time before I know which it is. I compared it to the difference between a tune, and a fugue theme. A fugue theme can be pretty simple, but it can be inverted, stretched, modified, and played against itself in a variety of ways. Even as you add things to make something a novel, the additions in some way reflect that underlying fugue theme.
Not a piece of practical advice, really ("But that's just a metaphor. I still don't know how to tell!") You'll have to ask one of my other panelists for something actually useful to you.