Add this to functions php to create image size hints with difference sizes for different post types From https://premium.wpmudev.org/blog/never-forget-your-featured-image-dimensions/

featured_image_size_hints_variable.txt

/* Featured Image Size Hints */
function karvldigital_image_size_html($content =''){
  global $post_ID;

  $post_type = get_post_type($post_ID);

  if ( ! ( post_type_supports( $post_type, 'thumbnail' )
    && current_theme_supports( 'post-thumbnails', $post_type ) ) )
  return $content;

  switch($post_type) {
    case 'post' : $dim = ' (440w × 300h)'; break;
    case 'page' : $dim = ' (600w × 200h)'; break;
    case 'testimonials' : $dim = ' (250w × 250h)'; break;
    default : $dim ='';
  }

  $content = str_replace(esc_attr__( 'Set featured image' ), esc_attr__( 'Set featured image' ) . $dim, $content);
    return $content;
}

add_filter( 'admin_post_thumbnail_html', 'karveldigital_image_size_html', 9999, 1 );