Check the size on paper.

How to use this sheet

1. Open the print dialog below and choose your loaded paper size.
2. Use 100% or Actual Size; turn off Fit, booklet and multiple-pages-per-sheet options.
3. Keep normal printer margins. Preview should contain one sheet without the site navigation.
4. Measure from the center of one end tick to the other, horizontally and vertically. Each long line should be 100 mm.

The page defines lengths in millimeters, but the browser or printer can still scale them. The physical measurement is the check; the screen is not a ruler.