This is what solved the problem of limited text input for comments in discussions.

Studio > Developer > Forms > Drop Down to System > In System Name column, you will see Comment. Click on 2 displays link under Displays column for Comment. You will see Comment Post Form > Then in Fields Column, click on 7 Fields. You will then see System Name column...scroll down to Text and click the Pencil icon to edit the field.

 Change Check for from Data Length to Data Availability. It will by pass checking character count and only checks to make sure that at least 1 character was entered and cannot be blank. It will no longer check to make sure you are not over 5000 characters.

See attached images. Set everything as marked in the images.

Once you access the Edit "Test" Field dialog box make sure everything is set as per the images attached.