if( ! function_exists( 'videotube_pull_full_size_post_thumbnail' ) ){
    /**
     * Pull full thumbnail size of the video post.
     * @param string $size
     * @return string
     */
    function videotube_pull_full_size_post_thumbnail( $size ) {
        global $post;
        if( get_post_type( $post ) == 'video' ){
            $size = 'full'; // or large
        }
        return $size;
    }
    add_filter( 'post_thumbnail_size' , 'videotube_pull_full_size_post_thumbnail', 100, 1 );
}