Unfortunately, you cannot set a character limit on the dynamic comment box in the survey. As a workaround, you can add a separate comment box question and use show/hide question logic on it, as character limits can be set for Comment Box questions.
Helpfile: https://www.questionpro.com/help/show-hide-question-logic.html
Comment Box question helpfile: https://www.questionpro.com/help/comment-box-question-type.html"