[jobananada]

<script>
let mass= [s,l,x,xl,xxl];

for(){
<div class="product-sizes">
	<span class="product-size selected" data-product-size="s">S</span>
	<span class="product-size" data-product-size="m">M</span>
	<span class="product-size" data-product-size="l">L</span>
	
	<select id="ProductSize" name="size" size="1">
		<option value="s">S</option>
		<option value="m">M</option>
		<option value="l">L</option>
	</select>
</div>
}
</script>

https://jsfiddle.net/BaNru/0z72a91u/3/ вот вариант без цикла