Load different post_thumbnail for smaller window width (jQuery,WordPress)

You cannot just create .html files arbitrarily, WP won’t recognize them and any PHP code won’t work in html file in first place (typically).

While you could bend this into working with some effort it would be bad approach since making Ajax request to WP will cause another core load and essentially double the load on the server (per each image).

The normal WP practice to pass data to JS script is to use wp_localize_script() for it. You can generate sizes in advance and that data will load together with page itself so script can immediately make use of it, without any extra requests.