I recently added two elements to a Word doc I have called "Story Starters". Under each story start (ranging from one sentence to three paragraphs) are several notes to myself. These include: where (as in where I originally recorded the start), genre, general assessment, use, and estimated word count. In determining the last of these, I found that I only needed four sentences of text (and my other notes) to feel comfortable estimating word counts. Since I haven't written them yet, I cannot guarantee accuracy, but I believe most of them will end up pretty close.
The cast of "Writing Excuses" seems to have some trouble getting a good estimate for their ideas. This is typically larger scale, as in novel vs. novella vs. short, but the same principles apply. Mary Robinette Kowal has a rule-of-thumb for herself: each character or scenic location adds 500-1000 words, although she didn't sound too sure of herself when she mentioned it on the podcast.
In order to estimate story length from an idea, you have to really know your own writing. The same idea for me will probably generate a story of a different word count than someone else. Each writer has his/her own style. Mine tends to be abrupt. I try to have my conflict evident in the first paragraph, preferably the first sentence. Some are subtle like my opening to "Xenicena": "Xenicena darted to her left". Others lay things on the table like the first paragraph of "All-Natural Warfare". From these openings and my knowledge of how I write, I could give decent estimations right away of my length. Of course, I suppose attention-span has something to do with it too...
Are you skilled at estimating the length of your stories from rough ideas?