Great question — answering as the UphillTech team:
It looks like you're missing a piece to the puzzle. Judging by the Mr Parker demo store, it looks like this functionality is already built into the theme. http://mr-parker.myshopify.com/collections/all Please note that you can only filter collections by tags, not by product variants. So in order to filter the collections by size you will need to go through and tag all of your products with the necessary values (e.g. s, m, l etc). As I haven't personally seen the theme code for Mr Parker I can't say for sure how it works, but it sounds like you will need to define these extra tag select menu either in the theme settings or maybe in a linklist.
Full answer by luciddarryn: https://stackoverflow.stackexchange.com/a/22158763 (CC BY-SA 4.0).
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