I dont know if this will exactly work for you then, was not realizing that it was a rich text box. But you can try this.
(choose this in the first box)The expression (This goes in the second) string-length(.) > 500 ((Set 500 to whatever character limit))
basically in your condition set it like this:
"The expression" "string-length(.) > 500" (and change 500 to whatever you want)