Add class to before_widget from within a custom widget

function widget( $args, $instance ) {
  extract( $args );
  ... //other code

  $widget_width = !empty($instance['widget_width']) ? $instance['widget_width'] : "col300";
  /* Add the width from $widget_width to the class from the $before widget */
  // no 'class' attribute - add one with the value of width
  if( strpos($before_widget, 'class') === false ) {
    // include closing tag in replace string
    $before_widget = str_replace('>', 'class="'. $widget_width . '">', $before_widget);
  }
  // there is 'class' attribute - append width value to it
  else {
    $before_widget = str_replace('class="', 'class="'. $widget_width . ' ', $before_widget);
  }
  /* Before widget */
  echo $before_widget;

  ... //other code
}

// source - https://wordpress.stackexchange.com/a/19052