class instaslidewidget extends WP_Widget
{
    public function __construct()
    {
        parent::__construct(
            "text_widget",
            "Insta Slide Widget",
            array("description" => "This is slide widget for photos from intagram")
        );
    }

    public function form($instance)
    {
      $width = "";
      if ( ! empty($instance)) {
          $width = $instance["width"];
      }
    }

public function widget($args, $instance)
{
    $width = $instance["width"];
    // ....
}

.rslides {
   position: relative;
   list-style: none;
   overflow: hidden;
   width: <?php echo $width; ?>;
   padding: 0;
   margin: 0;
}