ISBN

I don’t see the ISBN anywhere in the frontend area.

It’s not anywhere in the frontend area. However, the WordPress search has been altered so searches by ISBN number will return valid book results. If you need the ISBN to be listed in the review, you can add it to the content at this time.

Also how we can set the preview cover size?

If you are referring to the thumbnail images, that is set by your theme and/or WordPress. You can change the thumbnail size from the Media Settings page in the WordPress admin.

Via coding, is there a way to display the ISBN ?
At the admin area you have create it, just need to echo it at frontend, please advise.

Sure, you can add it to the template file for book reviews:

if ( get_post_meta( $post->ID, 'isbn', true ) ) {
    echo '<strong>ISBN:</strong> ' . get_post_meta( $post->ID, 'isbn', true );
}