/**
 * Implements hook_field_settings_form().
 */
function {{ machine_name }}_field_settings_form($field, $instance, $has_data) {
  $settings = $field['settings'];
  $form['max_length'] = array(
    '#type' => 'textfield',
    '#title' => t('Maximum length'),
    '#default_value' => $settings['max_length'],
    '#required' => FALSE,
    '#element_validate' => array('element_validate_integer_positive'),
    '#description' => t('The maximum length of the field in characters. Leave blank for an unlimited size.'),
  );
  return $form;
}