I'm not a published fiction author. My background has been writing for magazines and online in the area of travel and outdoors. I electronically published my first short story, The Huntsman's Tale, on Amazon and Smashwords on June 15, and have done a low to medium amount of advertising on it. This particular short story is just over 9K, although the one I'm currently working on will be around 6K, so for me, the length of the story really depends on the story idea. The genre of the story is fantasy- more specifically fairytale retelling.
I haven't had any B&N sales yet. Here's my Amazon data:
June 2011 - 3
July 2011 - 6
August 2011 - 6
I plan to release my second short story later this month, and I am genuinely interested to see how that will affect sales, although my second story is more science fiction. My general feeling towards writing short stories, is to write to the length that the story calls for. Right now at least, i'm trying not to worry too much about sales numbers, and hopefully just build up a readership.