Charles N. Brown, publisher and editor-in-chief of Locus, writing in the February 2009 issue, offers this understanding of the length categories for fiction, which I found illuminating:

"I have a bone to pick with the way people count the length of works, and more generally with the way stories are written. There are actual differences between the short story, novelette, or novella, more than word count alone: short stories generally focus on a single character and event, often within the Aristotelian unity of place, time, and action (i. e., plot). Novelettes generally have two characters, and novellas show the interaction between four characters (although one of them could be the world, the setting). As works get longer, they have a correspondingly larger story arc, greater thematic development, and the characters change more. Subplots generally don't enter in until the full novel length. This used to be the way the categories were understood, and can usually be accomplished best if we redefine short stories to be up to 10,000 words; novelettes 10,000 to 20,000, and novellas 20,000 to 60,000. Since the advent of the word processor, everything is just getting more bloated. I rarely see a story of any length that I don't want to take a red pencil to."

It occurs to me that this is a useful bit of guidance to analyzing a story idea before I begin to write, to get a sense of what its length (and hence pace) should be.