WordPress Widget - Show large form in the widget admin window

mywidget.php

class My_Widget extends WP_Widget {

	public function __construct() {
		parent::__construct(
	 		'mywidget', // Base ID
			'My Widget', // Name
			array( 
                          'description' => 'This widget will show a large form',
                          'class'       => 'mywidget-class' 
                        ),// Args
			array('width' => 400) // There is the trick
		);
	}

 	public function form( $instance ) {
		// outputs the options form on admin
	}

	public function update( $new_instance, $old_instance ) {
		// processes widget options to be saved
	}

	public function widget( $args, $instance ) {
		// outputs the content of the widget
	}

}
register_widget( 'My_Widget' );