In all honesty, I did not put a real limit on this because I do not want to hurt anyone's chance of winning by forcing them to omit details or ideas. So, technically, you can make it as long as needed. But yeah...15k words is a lot. The longest one I have so far is 3862 words (word count--I obviously did not count it myself), and it's pretty long. So, if you go over 15k words, it's probably too long.
On a side note to everyone, please send future stories in some sort of word document format (like MS word); really, anything but being typed in a PM.