"Regenerate Sizes" Link for a Single Image

add_filter( 'media_row_actions', function ( $actions, $post ) {
    if ( wp_attachment_is_image( $post ) && current_user_can( 'edit_post', $post->ID ) ) {
        $actions['wpide_regen'] = sprintf(
            '<a href="%s">Regenerate sizes</a>',
            esc_url( wp_nonce_url( admin_url( 'admin-post.php?action=wpide_regenerate_image&attachment=' . $post->ID ), 'wpide_regen_' . $post->ID ) )
        );
    }
    return $actions;
}, 10, 2 );

add_action( 'admin_post_wpide_regenerate_image', function () {
    $id = isset( $_GET['attachment'] ) ? absint( $_GET['attachment'] ) : 0;
    check_admin_referer( 'wpide_regen_' . $id );
    if ( ! current_user_can( 'edit_post', $id ) || ! wp_attachment_is_image( $id ) ) {
        wp_die( 'Not allowed.', 403 );
    }

    $file = wp_get_original_image_path( $id );
    $ok   = false;
    if ( $file && file_exists( $file ) ) {
        require_once ABSPATH . 'wp-admin/includes/image.php';
        $meta = wp_generate_attachment_metadata( $id, $file );
        $ok   = ! empty( $meta ) && ! is_wp_error( $meta ) && wp_update_attachment_metadata( $id, $meta );
    }

    wp_safe_redirect( add_query_arg( 'wpide_regen', $ok ? 'done' : 'failed', admin_url( 'upload.php?mode=list' ) ) );
    exit;
} );

add_action( 'admin_notices', function () {
    if ( isset( $_GET['wpide_regen'] ) ) {
        echo 'done' === $_GET['wpide_regen']
            ? '<div class="notice notice-success is-dismissible"><p>Image sizes regenerated.</p></div>'
            : '<div class="notice notice-error is-dismissible"><p>Could not regenerate this image: the original file is missing.</p></div>';
    }
} );