if( ! function_exists( 'videotube_filter_video_thumbnail_size' ) ){
    /**
     *
     * Filter the thumbnail image size
     *
     * @param string $size
     * @return string
     */
    function videotube_filter_video_thumbnail_size( $size ) {
        $columns =  mars_get_columns();
        switch ( $columns ) {
            case '3':
            case '4':
            case '6':
                $size= 'video-featured';
            break;
        }
        return $size;
    }
    add_filter( 'video_thumbnail_size' , 'videotube_filter_video_thumbnail_size', 100, 1 );
}