The easy answer to this question is: as long as it should be. I personally feel that you should write without a limitation in mind, and whatever the word count becomes is what is becomes. The story is the most important aspect. However, there are many short story / novel submissions that may ask for a certain word count to be observed. When I had finished writing the story The Box, the word count came to approx. 14000 words. When I checked short story submissions (excluding flash fiction), they ranged from 2000 words up to 10000 words (although 10000 is quite rare). When I checked novella submissions, I found that there are very few publishers and agents that entertain them, and I couldn’t find any journals or websites that would entertain them either. Another negative against The Box was the fact that novellas are usually around the 17000 to 30000 word mark, which left my story in a grey area between short story and novella, and this made it less likely to be published through an official channel. If you have written short stories and they clock up 5000 words instead of, for example, 2000 words, then try to edit your story down to what the submission guidelines require. If that fails, write another story. For novels there are debates over a benchmark word count. Some say 40000 words. Some say 70000 words. Some say a novel has to be 100000 words. The Reluctant Fundamentalist by Mohsin Hamid is around the 40000 word mark, A Clockwork Orange by Anthony Burgess is close to 60000 words, and Cloud Atlas by David Mitchell is close to 164000 words. All novels were well received and have had movies made out of them. The main reason for writing is to enjoy it and not to box yourself into a corner where you pressure yourself. If you started writing a short story and it becomes longer than you intended, then don’t worry because after all it’s the end result, the story, that is the most important – and you always have an option to publish it yourself.