Featured Image Size Hints where all featured images are the same size. From https://premium.wpmudev.org/blog/never-forget-your-featured-image-dimensions/

featured_image_size_hints.txt

/* Featured Image Size Hints */
add_action( 'do_meta_boxes', 'karveldigital_do_meta_boxes' );
function efs_do_meta_boxes( $post_type ) {
  global $wp_meta_boxes;
  if ( ! current_theme_supports( 'post-thumbnails', $post_type ) || ! post_type_supports( $post_type, 'thumbnail' ) )
  return;

  foreach ( $wp_meta_boxes[ $post_type ] as $context => $priorities )
  foreach ( $priorities as $priority => $meta_boxes )
  foreach ( $meta_boxes as $id => $box )
  if ( 'postimagediv' == $id )
  $wp_meta_boxes[ $post_type ][ $context ][ $priority ][ $id ]['title'] .= ' 215w x 139h';

remove_action( 'do_meta_boxes', 'karveldigital_do_meta_boxes' );
}