Short stories are, in general, works of fiction between 1,000 and 25,000 words, though the distinction varies between writers, critics and academics.
Works less than 1,000 words are defined as MicroFiction. Works between 25,000 and 40,000 words are novellas and over 40,000 they are novels. Very long novels (120,000 words plus) may be called epics, sagas or something similar.