The short story is a form of narrative prose writing that is characterised by the number of words contained therein.
Determining the actual length of a short story is problematic. A classic definition of a short story's length is that it must be able to be read in one sitting but in contemporary usage the term most often refers to a piece of fiction up to 20,000 words. In practice, however, a short story's length is determined by where it is published. In the United States, for example, short stories can be anything up to 10,000 + words (and are called “long short stories”) whereas in the United Kingdom short stories average around 5,000 words but in Australia they are rarely more than 3,500 words. Although some short stories can be just a few hundred words long (and are called micro narratives) there is an expectation among contemporary readers that short stories are at least 1,000 words in length.
Source = Wikipedia