In several sets of UX guidelines from device manufacturers, there is often a minimum size for UI elements, such as the 48 CSS pixels suggested by Google also offering a physical measurement of around 7mm.
My only issue with this is the fact that the relationship between CSS Pixels and physical measurements such as inches or millimetres is very blurred between all of our devices.
Growing up with a dad who working in printing and myself building websites since the late 90s, I've straddled a world of many different measurements from picas to pixels. Along with the ever complicated idea of resolution, whether you talk of DPI or PPI.
Would moving back to using physical world measurements when designing for touch screen based devices make more sense than designing using pixels? Providing of course that each device/browser can render these correctly.