Field module in Drupal core decides you are not allowed to change certain field settings once there is field data stored. Sometimes it's wrong.

The dgd7formoverride module, available in the definitivedrupal project's codebase, was created to unlock the taxonomy vocabulary forms which needed to be corrected after upgrading from the dev and alpha releases.

I believe the same approach could work to allow making the maximum size of a text field larger, but have not tested that.