Due to limitations in the information that Shopify makes available to Themes when we are displaying your collection page, themes are only able to filter your collections by Tags.
While it is possible to add Tags for each size or price range you'd like to show in a collection filter, it is often too cumbersome for most merchants to add and remove those tags as sizes go in and out of stock. For price ranges, creating and linking to price-automated collections may work for your situation (ex: an "Under $50" collection).
If filtering by size stock levels and price ranges is a requirement for you to run your business effectively, your best bet is to use a filtering app like Boost Commerce's Product Filter + Search which we've seen integrate well with our themes.
Hope that helps! If we could, we'd have built this into our themes already :)