if( ! function_exists( 'videotube_filter_post_thumbnail_size' ) ){
    /**
     *
     * Retrieving full thumbnail image for video posts
     *
     * @param string $size
     * @param string $post_id
     * @return string
     *
     * @since 1.0.0
     *
     */
    function videotube_filter_post_thumbnail_size( $size, $post_id ) {
        if( get_post_type( $post_id ) == 'video' ){
            return 'full';
        }
        return $size;
    }
    add_filter( 'post_thumbnail_size' , 'videotube_filter_post_thumbnail_size', 9999, 2 );
}