Minimum comment length?
Add a custom validator to form using hook_form_alter in your custom module.

function custom_mod_form_alter(&amp;$form, &amp;$form_state, $form_id){
  switch($form_id){
    case 'comment_form':
      $form['#validate'] = array_merge( $form['#validate'], array('custom_validator'));
    break;
  }
}


Define the logic to restrict the length of comment length

function custom_validator($form, $form_state){
    if(strlen($form_state['values']['comment']) &lt; 50)
       form_set_error('comment', t('No No....'));
}