There have been attempts to define physical units (Firefox used to have a unit called mozmm which tried to be one physical millimetre), but eventual consensus is that (a) you can’t actually figure this out, because screens don’t tell you their exact physical sizes; (b) if it’s not accurate, there are exactly zero genuine use cases for it; and (c) even if it was accurate, it would still almost never be correct to use it. Remember that you don’t know the viewing distance, either—letters being 5mm high is all very well twenty centimetres away, but at four metres it’s utterly illegible.
