You'd have to analyze the code to see how they're being displayed, but if I were doing this here would be my approach:
1) Read the code to see how they're currently displayed. I'd guess this is in a Snippet called sidebar.liquid, but if it's on the collection page you could start with the Section for the collection, maybe collection.liquid, then trace the code from there to find that area.
2) See what the name of the option is (maybe "Size"), and if that option, then put into an array and sort (https://shopify.github.io/liquid/filters/sort/)
3) Preview various collection pages to make sure everything looks as expected.
Always work on a backup theme when making code changes. If you prefer a developer to handle this, feel free to contact my team at speedboostr.com/contact. We do website edits like this all the time.