You could add the available sizes as tags to each product. If you prefix the tag with "size_" you can easily make a dropdown menu with all available sizes within a collection.
So you don't need an app, but you need to customise your theme, let me know if you need help with that.
Can't see if you already have a solution, as the shop is closed now.
There's an easy to use app for this, which will auto-tag products for you: https://apps.shopify.com/collection-filter
Hope it helps.