RWMB_Select_Field::normalize PHP Method

    public static function normalize($field)
    {
        $field = parent::normalize($field);
        $field = $field['multiple'] ? RWMB_Multiple_Values_Field::normalize($field) : $field;
        $field = wp_parse_args($field, array('size' => $field['multiple'] ? 5 : 0, 'select_all_none' => false));
        return $field;
    }

 /**
  * Normalize parameters for field
  *
  * @param array $field
  *
  * @return array
  */
 static function normalize($field)
 {
     $field = wp_parse_args($field, array('js_options' => array(), 'placeholder' => 'Select an item'));
     $field = parent::normalize($field);
     $field['js_options'] = wp_parse_args($field['js_options'], array('allowClear' => true, 'width' => 'off', 'placeholder' => $field['placeholder']));
     return $field;
 }