Enhancement: Display Notice for Featured Image's Recommended/Required Size/Dimensions

Expected behavior

If a theme requires featured images to be a certain size and a user attempts to upload an image outside of that size, then they are usually notified via a notice in the post editor's sidebar. Notices around recommended sizes for featured images are also sometimes incorporated in SEO plugins, for example, the official AMP plugin.

I'd expect similar notices to be displayed within the app when appropriate, especially in cases where the requirement is coming from the theme and determine whether a featured image actually displays.

Actual behavior

No such notices are displayed when a user uploads featured images via the mobile app. Although this doesn't cause any outright harm, it does mean app users may miss out on useful defails that may help with their site's appearance in search results and on the web.

Steps to reproduce the behavior

• Log into WordPress.com in a web browser and select any site with AMP enabled. Note, the site will need to be public and you can enable AMP under Settings → Performance.
• Navigate to Posts → Add New.
• From the Featured Image panel in the sidebar, attempt to upload any image that's below 1200px in width.
• Notice the following message: The featured image should have a size of at least 1200 by 675 pixels:

• Next navigate to the same site/post within the mobile app, via My Site → Blog Posts.
• Navigate to post settings and, again, attempt to set a featured image that's less than 1200px in width.
• Notice how no message is displayed.

iOS version of report: https://github.com/wordpress-mobile/WordPress-iOS/issues/16150

Start in the mobile app's post settings and featured image upload flow described in the reproduction steps. Compare the app's behavior with the web editor for theme- or plugin-provided dimensions, including the 1200 by 675 pixel example. Done means an appropriate size notice is displayed when a featured image does not meet the applicable requirement.