comment char/word limit plugin

Does anyone know of any plugins that limit the number of characters or words a user can type? I know this may be an atypical request, since most people are more than happy to get comments, regardless of length, but I want to curb the 500-word essay comments.

Example of what I’m looking for (see comment box):
http://www.stylegala.com/news/200512/711.htm#comments