This is was a quick little console script I wrote to help me regenerate the thumbnails of only the posts or in this case products specified.

console.js

// HTML: add the tag: <iframe id="f" src="" width="900" height="600"></iframe>
// Then run the following in the console:

var
	// Array of wp_posts ID's that need their thumbnails regenerated
	ids = ["328921", "326496", "322847", "21967", "16180", "16173"],
	
	// Prevent handling the onload event at the wrong time
	loaded = false,
	
	// Begin editting a new product
	run = function (id) {
		loaded = false;
		document.querySelector('#f').setAttribute('src', 'https://yourwebsite.com/wp-admin/post.php?post=' + id + '&action=edit');
	};

// The product has loaded begin the regen.
document.querySelector('#f').onload = function () {
	if (loaded)
		return true;
	
	loaded = true;
	
	// Click Regnerate Photos
	document.getElementById('f').contentWindow.document.getElementById('sis_featured_regenerate').click();
	
	// If there are more ID's left to go then wait for the regen to finish first
	if (ids.length) {
		console.log(ids.length + ' more to go');
		var chk = function () {
			// If this is blank then the rengeration is not complete
			if (document.getElementById('f').contentWindow.document.querySelector('div.sis_message').innerText === '') {
				setTimeout(chk, 500);
			} else {
				// Products thumbnails have regenerated run the next id
				run(ids.pop());
			}
		};
		
		// Check to see if the regeneration has completed
		setTimeout(chk, 500);
	}
};

// Init
run(ids.pop());