Use the script to set a special sorting order to the size filter
<script>
var __isp_options = __isp_options || {};
__isp_options.isp_serp_size_sort_order = ['extra small', 'small', 'medium', 'large', 'extra large', 'extra extra large', 'xxs', 'xs', 's', 'm', 'l', 'xl', 'xl/1x', '1x', '1x/2x', 'xxl', '2x', '2xl', '2x/3x'];
</script>