Most of my short stories I've thought up out of the blue and written without deciding how long they were going to be beforehand. This is fine if you're writing for yourself or are so established that you are getting collections published.
Most magazines or competitions have rules though. You're supposed to write on a theme or at least stick to a word limit. When I started writing I hated that concept. I wanted to be free to write any stories I wanted, and make them how short or how long I wanted them to be.
Now I find it quite exciting to brain-storm around a theme. It takes me to places I didn't know before. It makes me write things I wouldn't normally have written.
I've been working on a story for almost two weeks, inspired by an exercise in the excellent book Short Circuit - a guide to the short story, edited by Vanessa Gebbie. The exercise was about setting. First you were supposed to describe a room in your house, pretending you saw it for the first time. Then you were supposed to place a person in the room whose mood reflected the mood of the room. Lastly you were supposed to introduce another character and through dialogue reveal their relationship to each other. I combined this exercise with an idea I got from reading a message on my radiator, left by a previous lodger.
I have no idea where to send this story, but a lot of competition/magazines have a word limit of 2000 words so I decided to aim for that. When the story was finished I had 2350 words. I had no idea how to cut it, but leaving it a few days and getting back to it, I could easily shorten the story to 1950 words. The advantage with a word limit is that you learn how to cut out the crap. At least if you're a novelist like me and are used to waffling and having space. It might be harder if you're a flash fiction writer and feeling forced to add words ... (Normally there's no lower word limit, but I suspect it's hard to win a competition with a 300 word piece. There a exceptions though. Tania Hershman is a master when it comes to short short stories.)
Next story I'm going to work on is for the Asham Award. The word limit is 4000 words, which is not a problem. The problem is that, unlike other years, they have a theme this year. And the theme is ghosts. I've never written a ghost story in my life, so this will be a real challenge ...