Nathan, I fought tooth and nail quite a while back for a higher word limit. I got 750 (or was it 800?). I asked for it for structural reasons. We were recreating fairytales. For a challenge based on a simple one line prompt I think you'll be hard pressed to convince anyone it needs to be higher. And if so, you need to convince the judges. If there's four judges willing to judge a higher word count then I guess you'll be in luck.