Much like the animal kingdom, literature has been broken down into neat little categories so that way we may all rest easy at night knowing that the world is in order. The five major categories for literature are:
An interesting thing to note is that "short story" is not only it's own category, but it's also one of the major categories. Contrary to current popular belief, a short story is not, in fact, a sub-genre of the novel. Nor is it a novel cut short. A short story is its own clearly defined approach to literature.
So why then do we rate them by a novel's standards? All too often the only criticism of a short story will be "it was too short, waste of money." Of course it was short, hence the category. A waste of money? I suppose that is a matter of opinion. If number of words is vitally important to you as a reader, then yes that would be a viable concern. In order to get the most bang for your buck, it would be best if you did not buy a $4.99 paperback copy of Peter Pan (too short!) and instead went with the $0.99 ebook version of War and Peace.
Or we could think about what it is that we actually want to read. Reading should be a pleasure. If the summary of a story looks interesting, why not try it? Number of words is never an issue for any of the other categories of fiction. People usually go for the novel or set of poems that appeals to them; trying to find the cheapest, longest book that money can buy is almost never a priority. It is important to give all the categories of fiction a fair trial before passing judgement.