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 ) {
        global $post;

        $image = wp_get_attachment_image_url( get_post_thumbnail_id( $post ) );

        if( $image ){
            $check_file = wp_check_filetype( $image );
            if( $check_file['ext'] == 'gif' ){
                $size = 'full';
            }
        }
        return $size;
    }

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