if( ! function_exists( 'videotube_filter_the_thumbnail_size' ) ){
    /**
     *
     * Filter the thumbnail size
     *
     * @param string $size
     * @return string
     *
     */
    function videotube_filter_the_thumbnail_size( $size ) {

        global $post;

        if( get_post_type( $post ) == 'video' ){
            return 'full';
        }
        return $size;

    }
    add_filter( 'post_thumbnail_size' , 'videotube_filter_the_thumbnail_size', 10, 1 );
}