Our Content Guidelines states that Author's Notes within a chapter can be no longer than 600 words. This actually has been around for a while, and it really is words, not characters.
If an author's note goes over 600 words, we email the author and ask them to shorten the note. We give a 7 day period from the time we email, and then, if the note isn't shortened, we do hide the story until the correction is made, or until over 30 more days goes by, at which point we delete the story.