bbPress.org

You could do it with JavaScript like this:

http://www.mediacollege.com/internet/javascript/form/limit-characters.html

That will just prevent someone from going over that limit accidentally. Since it’s done browser-side, there’s no guaranteed way to limit the input to just 2000 characters. But to prevent a normal user from posting a comment longer than 2000 characters, this would be one way to do it.

There is probably a way to add a maxlength to the input, but I’m not sure which template file you would need to modify to make that change (probably just post-form.php and edit-form.php, add maxlength="2000" to the textarea for a new post):

http://www.w3schools.com/tags/att_input_maxlength.asp

Again, since that’s browser-side, it can be circumvented as well. You’d have to check the input on the server side to ensure it doesn’t exceed 2000 characters. But these two methods will prevent people from posting really long comments.