I don't really count how many words I write in each story, the number of words that are put in them is just how I evolved as a writer. My first story is only like 1,300 words because it was my first try and there isn't a plot to it really. The last story on here is over 10,000 because of how much more plot, detail, and basically more thought I put into it.
Deciding on writing a new short story is hard, Ideas for a new story will not always come when you want it. I haven't written anything recent because I haven't come up with any ideas that I think would work out. So what I'm saying is I don't really decide when I'm going to write/post, it just when I get an idea that works then I'll write a new story. I hope that answered your question(s).