The white square is sized for about 50 mm per side on glass (ruler check). Measure the outer white area. Scanning opens openstrate.com/#calibrate?size=50.
On this screen width, the white square is 100 mm per side (measure the outer white area). The page tries to stay true to mm when you use Ctrl+/− zoom; if the ruler still drifts, reset zoom to 100% (Ctrl+0). Scanning opens openstrate.com/#calibrate?size=100.