The physical size is always wrong on the screen, e.g. the rulers show dimensions which do not measure the actual dimensions on the screen when zoom is set to 100%.
I ran the following experiment:
xrandr --dpi 100
xdpyinfo | grep -B2 resolution # confirm it says "100x100 dots per inch"
/path/to/libreoffice/program/soffice --writer
Set zoom to 100%
Verify that Page format is 8.5x11" (US Letter)
Measure width of paper on screen: About 7 5/8" (should be 8 1/2")
Then repeated the above use used xrandr --dpi 200 i.e. doubled the DPI.
Libreoffice displayed the page at exactly the same physical size.
What’s going on? How does Libreoffice determine the DPI of the screen when running under X.org, or maybe I should ask how does Libreoffice decide how many pixels the page image should be displayed at?