I’d like to sort my display and search filters - particularly the size filter to have the numbers (8-20) to always be at the top
If you have the size containing only a number then it will be sorted auto-magically. If not then you might need to create some code logic to render it based on the number inside the value. Or just try to manually add it. I personally adding to manually with this app https://apps.shopify.com/ultimate-search-and-filter-1