How can I filter by variant. I mainly want to fitler by size on my site. I understand I can get an app and pay monthly forever (which I hate). I just seems to me this should be added into the navigation in shopify! I know some themes filter by tags. I do not know how to tag a single variant. If I did it is only duplicate work, because it is already a variant.
@rainwater - As you mentioned, tags are at the product-level (not variant) so there is no path to go down there. We've implemented filter solutions that use the variant info already keyed in (ie. option names and values) but the development varies for each use case so there is no "one-size" solution that can be offered on the forum.
Regardless, this requires a fair amount of development which is why some businesses opt to go with the app approach. Not necessarily with the intent to pay forever, but instead to gain a better understanding of how their users interact with a filter prior to spending the capital on developing a custom solution. Kind of a "try before you buy" mentality.
Either way, we would recommend working with a Shopify Expert to run through costs and timelines associated with the different avenues. Either way, Hope this helps!