From what I understand, the shopping UI is going to live in a side bar. For consistency with other side bars (e.g. History and Bookmarks) and better OS integration and accessibility, it should probably use the system font size (i.e. set the system-font-size class on the root element) or provide a way for users to zoom in and out, as the standard zoom UI for web content isn't available for side bars and I believe default zoom and minimum font size settings from about:preferences don't apply either.

Thanks for filing this bug. For reference, the figma is here: https://www.figma.com/file/Wu4kaHSfKSQnzkym7qek3R/

If you're interested in learning more or asking questions, I'm happy to chat async or on zoom.

Setting system-font-size makes the text way smaller than the design specs, unfortunately. The existing sidebar also messes with system font size for its elements, at least on macOS, because the font is too small otherwise. So it's not clear to me how this should move forward without more UX input. I think given the limited scope of the experiment, we should do this once we finish building the initial experience.