Spooky. I've just been discussing the origins of the centimetre in an unrelated context. My opinion is that they're something the devil came up with on a slow day, to sow a little extra confusion and consternation amongst mortals. And I have some heavyweight backing for that from the Bureau International des Poids et Mesures, who regard the cm as a "non-preferred" unit. No engineer will have anything to do with them.
British rulers are a bit different, or at least there's quite a variety. Tenths of an inch are seldom seen outside of an engineering context, and the commonest division is into eighths and sixteenths. Good steel rules generally have hundredths for part of their length. However, Imperial length measurements are rarely used in industry, and the standard units are metres and millimetres: metric steel rules usually have whole mm along one edge and 0.5mm along the other.
How this relates to calligraphy: well my usual x-height is 2mm simply because I generally use Brian Walker's guide sheets and that's the size he likes, with 12mm between baselines. This is not unduly small for Spencerian - 1/16" (1.6mm) is common in old examples and Michael Sull suggests this is a sensible size to work towards for business writing - but it's probably a good deal smaller than most copperplate writers would care to use.
Surely nobody has suggested writing at 1mm including ascenders and descenders - I thought we always talked in terms of distance between base and waist lines? Anyway, 1mm x-height is certainly doable, but not a whole lot of fun for anything more than a return address on an envelope.