Here's what you have to do if you want to increase the character limit for a "Contact Form" element:
Firstly, edit the page with the form using Thrive Architect:
Once the editor opens, select your form and click on "Edit Form Elements", to enter the "Edit Mode":
Select your "Textarea" field, and its options will appear in the left sidebar. One of the options here is the "Max. Characters" one:
You can modify this number by using the up/down arrows, or by entering a numerical value in the field:
You are also able to modify the minimum number of characters from here as well.
Click on "Done" to exit the "Edit Mode" when you're finished:
You're all set to save the page!