Set Helper Text on WP Featured Images

featuredImageFilter.php

<?php  
add_filter( 'admin_post_thumbnail_html', function($content, $post_id, $thumbnail_id) {
  if ( 'post_type' !== get_post_type( $post_id ) ) {
    return $content;
  }
  // remove the default recommended size
  $string = '<p class="hide-if-no-js howto">We recommend a 16:9 aspect ratio for featured images.</p>';
  $content = str_replace($string, '', $content);
  // add a custom caption
  $caption = '<p class="howto">' . esc_html__( 'Minimum image size: 783x645', 'iv' ) . '</p>';
  // return the content with the caption
  return $content . $caption;
}, 10, 3 );
?>