I don't think I'd want to do that, decide on how many words it's going to be and try to fit within that area. It would be too uncomfortable for me and take away all the fun in writing. In one of my english classes, we were taught not to focus so much on how long the story is going to be and more on just letting the words flow. A story is not an essay, so there's really no need to say "This has to be such and such words, how can I make it fit?", in my opinion. No offense.