4 mm graph paper, A4
Four millimeter squares on A4, a common exercise book ruling and a useful step below 5 mm.
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A real classroom ruling, not an arbitrary size
Five millimeters is the metric default and the one most people reach for, but 4 mm turns up throughout European school stationery, particularly in exercise books for mathematics where the work is dense. It fits about 48 squares across an A4 sheet inside sensible margins, against 38 at 5 mm, which is enough extra room to matter over a long working.
Small enough to change how you write
At 4 mm most handwriting spans two squares rather than sitting inside one, which is the point at which the grid stops being a container for characters and becomes a coordinate system. That suits algebra, where you want columns of working to line up, and it suits any layout task where the grid is a measuring device rather than a writing guide.
Lighten the grid as it gets finer
More lines on the page means more ink and more visual weight. At 4 mm the default line weight starts to read as gray tone rather than as individual lines. Drop the weight and lighten the color in the generator and the grid recedes to where it should be. This matters more the finer you go, and by 2 mm it is essential rather than optional.
Printing it accurately
This only measures correctly if your printer doesn't rescale the page. In the print dialog set Scale: 100%, not "Fit to page" or "Shrink oversized pages". If you're not sure it worked, print the calibration page, measure the ruler on it with a real ruler, and it will tell you exactly what your printer is doing to the page.