Show message next to featured image in backend of WP - add to functions.php

featured-image-backend-message.php

//
// Add Comment Next To Featured Image on Post Types
//

function add_featured_image_display_settings( $content, $post_id ) {
    global $post;
    if ($post->post_type != 'post') {
        $field_id    = 'show_featured_image';
        $field_value = esc_attr( get_post_meta( $post_id, $field_id, true ) );
        $field_text  = esc_html__( 'Recommended Size = 400px x 400px', 'generatewp' );
        return $content .= $field_text;
    } else {
        $field_id    = 'show_featured_image';
        $field_value = esc_attr( get_post_meta( $post_id, $field_id, true ) );
        return $content .= "";
    }
}
add_filter( 'admin_post_thumbnail_html', 'add_featured_image_display_settings', 10, 2 );