Canonical Way to Specify Readable text for TV feeds: Pixels?
Specifying text in terms of pixel size is the most independent way of specifying size. Where you know the vertical resolution (like 720 lines or 1080 lines) then specifying pixels is exactly like saying "a fraction of the total frame".

And in practical terms, 12 pixels is pretty much the minimum recognizable height for common western characters, especially if some viewers are watching on screens of lower resolution.