Dot grid paper, Legal size
Five millimeter dots on Legal paper, which is the same width as Letter and three inches taller.
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The extra height is the whole reason
Legal is Letter plus three inches of height, and for anything that runs vertically that is a meaningful gain: a long checklist, a timeline, a call log, a stack of meeting notes that would otherwise break across a page. At 5 mm spacing those three inches are about fifteen extra rows of dots.
Dots suit mixed content better than lines
A page that has to hold prose, a small table and a sketch is awkward on ruled paper, because the lines fight anything that is not writing. Dots handle all three without imposing. On a long sheet, where you are more likely to be mixing content types down the page, that flexibility is worth more than it is on a short one.
Set the paper size in the print dialog
Most home printers will take Legal but almost none select it automatically, and the standard failure is a driver quietly scaling a 14 inch page onto an 11 inch sheet. That compresses the spacing and you get a grid that is not 5 mm. Choose Legal explicitly, and if the sheet still comes out wrong, the calibration page will tell you what the printer is doing.
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.