Change #default_value of a textarea on admin pages
The teaxtarea form field doesn't have any property for the maximum length of the accepted text. The module showing the textarea can implement a form validation handler to limit the length of the entered value, and (generally speaking) that limit is not saved in a module settings; it is generally hard-coded.

In the case the value contained in the textarea field is saved in a database table created from the module showing the form, there is another issue: The value contained in the textarea field must be contained in the database field. If it is longer than what the database field can store, you would get SQL errors.