Is there a maximum size to form fields like text field?

Document ID : KB000100852
Last Modified Date : 11/06/2018
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What is the maximum character length for the Text Area and the Text Field in Catalog?
You can define maximum size for these fields when you build the form. 
So this can be whatever you set.
However for an overall top limit,
these form data values are saved in form_elem_value column in usm_request_item_form table .
This column is defined as ntext type in Microsoft SQL server . So the limitation will be ntext type limitation . 
This is a very large value so you will likely not hit the hard limit.