Change label max length on all field types
You can create a module and include the hook_form_alter for the content types you want. This is an example of how to do that:

function [MODULE_NAME]_form_alter(&amp;$form, $form_state, $form_id) {
  if($form_id == 'your_form_id'){
    $form['your_field_name']['label']['#maxlength'] = 255;
  }
}


Also you can try this module: http://drupal.org/project/remove_field_label_length_limit