Process data with the text datatype are limited to 4000 characters.
If you want to map a textbox with more than 4000 characters, you have to use two textboxes associated with two process data, for example.
See http://www.w3schools.com/tags/att_textarea_maxlength.asp for a solution to define a max length.
Note: The maxlength attribute is supported in Internet Explorer 10+, Firefox, Opera 15+, Chrome, and Safari.