Graph paper, A6
Five millimeter squares on A6, which is a quarter of an A4 sheet and about the size of an index card.
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A quarter of A4, and that is the useful part
A6 is 105 by 148 mm, exactly half of A5 and a quarter of A4, so four of them tile an A4 sheet with no waste and no awkward remainder. That makes a stack cheap to produce: print four up, cut twice, and you have a pack. The size is close enough to a standard index card that it works in the same boxes and pockets.
Small format, unchanged spacing
Keeping 5 mm on a small sheet rather than shrinking the grid is deliberate. Spacing should match your handwriting, not the paper, and shrinking the grid with the page gives you a sheet you cannot write on. At 5 mm an A6 card holds about 18 squares across and 26 down, which is enough for a diagram, a short calculation or a day's worth of small notes.
Where pocket grid earns its place
Field notes, quick dimension sketches, a running calculation you want to keep, flashcards with a diagram on them. The common thread is that the note is short and needs to survive being carried. If you want the same idea with dots instead of lines, the A6 dot grid is less visually heavy and suits sketching better.
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.