if( ! function_exists( 'videotube_pull_full_thumbnail_size' ) ){
    /**
     * Pull full video thumbnail image
     *
     * @param string $size
     * @return string full
     */
    function videotube_pull_full_thumbnail_size( $size ) {
        return 'full';
    }

    add_filter( 'video_thumbnail_size' , 'videotube_pull_full_thumbnail_size', 10, 1 );
}