Change #default_value of a textarea on admin pages I need to change specific textarea max-length (currently 100 letters). 

Where is the default value stored or how it is possible to override this setting?
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.