Add Minimum Dimensions Text to Featured Image

Post Thumbnail HTML

Content can be added to the post thumbnail box using the admin_post_thumbnail_html filter. This code can be added to a theme’s functions.php or a plugin. If the sizing guidelines are tightly coupled to the theme, I’d probably put it in functions.php. If you want this text to be present when switching themes, I’d go with the plugin approach.

add_filter( 'admin_post_thumbnail_html', 'wpse240830_admin_post_thumbnail_html', 10, 3 );
function wpse240830_admin_post_thumbnail_html( $content, $post_id, $thumbnail_id ) {
    $content .= __( 'Set featured image (837px x 467px)', 'your-text-domain' );
    return $content;
}

Creating a plugin

Creating a plugin is pretty straight forward. The Codex article Writing a Plugin and the Plugin Handbook are good places to start. In short, you just create a php file in the plugin directory which contains a special header. Put your code in the plugin file, then activate it.

<?php
/*
Plugin Name: WPSE Plugin
Plugin URI:  https://developer.wordpress.org/plugins/the-basics/
Description: Basic WordPress Plugin Header Comment
Version:     0.0.1
Author:      http://wordpress.stackexchange.com/
Author URI:  http://wordpress.stackexchange.com/
License:     GPL2
License URI: https://www.gnu.org/licenses/gpl-2.0.html
Text Domain: wpse
Domain Path: /languages
*/

// Exit if accessed directly
if ( ! defined( 'ABSPATH' ) ) {
    exit; 
}

// Add plugin code here.