Personally I don't think length matters. Once I came across a really good LBT short story. It was certainly not enough to be turned into a movie (as it mainly consisted of pondering of a character), but it sure didn't include anything that could not have been in a movie. Personally I think this list ought to contain land before time fanfictions of any length so long they don't include any elements which would be off limits in the movies (even if they provide too little or too much material for them to be turned into a movie).