Are there any other cartridges you know of, affordable or not, that use a stylus as small as the Zyx?The Lyra Olympos (discontinued) stylus was longer but it looked similarly narrow to my naked eye, but I never saw the specs. Current top Lyra's like the Titan might use something similar. Check VdH and Dynavector too.
The ZYX stylus is measured in a somewhat odd way because it's shaped in an odd way. If you cut a horizontal cross-section through the stylus at groove contact height, it would look something like an ellipse with a little ridge sticking straight out from each of the narrow ends. The end of this ridge is all that contacts the groove wall.
In the booklet that comes with the Airy 2, Airy 3 and UNIverse ZYX quotes the radius of these ridge ends as 3um, so 6um in diameter. The radii of the main part of the ellipse would be much larger, but it doesn't touch anything and the size isn't quoted AFAIK.