'Return the size of the user name field for the backend.
Subclasses can override this method to indicate that the user name
field is limited in length. The user name is the field used in the test
that validates that a filter value works even if it\'s longer than a
field.
If the backend doesn\'t limit the value length then return None.'

def _get_user_name_field_size(self):

    return None
