You can limit the number of characters entered into a Text Entry Box. The maximum number of characters that will now be accepted by a TEB in Captivate 9 is 150 characters. But you can set the number to be less than this.
You can certainly include a Text Entry Box in a quiz, but that is usually only when you have set the Validation option to ON so that the learner's text is evaluated to see if it matched a correct answer. In your case there does not appear to be any need for a correct answer if you just want feedback. If you do not set Validation to ON then the Report to Quiz option is greyed out.
Developers wanting to have course feedback accepted by the LMS often use the Short Answer question set to Survey instead of Graded. It seems that some LMSs will then record the entered text.
So it's a bit of a Catch-22. I still believe your best option is to simply tell the users NOT to enter more than a couple of sentences of feedback.