Recently, I've been writing a short story to submit to a publisher for a specialty anthology they'll be publishing in the summer. My first effort is a story I love so much, and the characters are so rich and situation so interesting, I had to rush the end to come in at the correct word count. After considering what I was doing and why, I decided to give that story the shot it wants at a longer version. I've come up with another story that should come in at the correct word count with no problems of the story getting out of control.
So, why should I decide to listen to what the story is telling me and begin writing a new story from scratch to submit instead? Because I'm a firm believer that the characters and situations should be what determine the length of the story, not some arbitrary yard stick we writers put down to measure length by. However, I do have a good idea when I start writing if I'm going to try for a novel, novella or short story. A lot of times I can tell by the ideas themselves if I can squeeze 90K out of it, or barely scrape by at 25K. A lot of times it depends on how intricate I get with subplots and backstory, or how complicated the conflicts. Me...I love a good complicated conflict.
So whether my heroes are slaying their foe to save the beautiful heroine like the Dicksee painting on the right, or traversing the galaxy in search of new worlds I want to give my story the opportunity to shine in whatever length they will best fit.
I'm just the conduit.