PLATOjapanese "medium" is thinner than american "medium". Look online for writing samples with it - it should be fine for writing on normal papers with the right ink.
PLATOI have one that I purchased a few weeks ago. Measuring the line width gives ~0.65mm with the current ink I have in it (Noodler's Black Swan in Austrailian Roses) on some scrap printer paper. Keep in mind that line width tends to vary slightly with the ink and paper choices, and how hard one presses. I also expect there is some level of variation between different nibs.