A short story is however long you want it to be. Wikipedia says they are generally between 1,000 and 9,000 words based on one definition.
For hubpages though you might want to consider flash fiction, stories which are typically less than 1,000 words (also according to Wikipedia).
Lewis Carroll said "Begin at the beginning and go on till you come to the end; then stop." This is what should determine what format it is in, whether it be flash fiction, short story, novella, novel, or series. Short stories can easily turn into novels, and sometimes long stories need to be shorted to flash fiction.