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Actually "NR 4.2.2 Setting the staff size" kind of document it: The default staff size is 20 points, which corresponds to a staff height of 7.03mm (one point is equal to 100/7227 of an inch, or 2540/7227 mm). " Thanks! Even more info on the units can be found "5.4.2 Distances and measurements". So there is actually quite some documentation. I personally lacked this information on the page "1.8.2 Formatting text – Selecting font and font size", mainly the fact that text-font-size is in pt (as defined in other sections). AS " "
