Graph paper, Legal size
Quarter-inch squares on 8.5 × 14 inch Legal, with a heavier line every inch. Three more inches of vertical run than Letter, and not a millimetre more width.
Opens with these settings already loaded. Change anything you like, then download as PDF, PNG or SVG.
Long, not large
Legal is Letter with three inches added to the height and nothing added to the width, which makes it useless for anything broad and genuinely useful for anything that runs downward: a timeline, a project schedule, a tall column of accounts, a flowchart that keeps branching, a geological or stratigraphic section. At quarter-inch squares you get 44 columns and 56 rows, against 44 by 44 on Letter.
Where the size still exists
Legal survives in North America — the United States, Canada, Mexico and the Philippines — and it is a professional habit more than a standard. Most printers hold it in a second tray if they hold it at all, and many drivers will cheerfully print a Legal PDF onto Letter and crop three inches off the bottom without saying anything. Check the tray as well as the dialog before you print a stack.
Turned sideways it is a wide sheet
In landscape you have fourteen inches of horizontal run, which is the cheapest way to get a genuinely wide chart out of an ordinary office printer. A Gantt chart, a year-long tracking grid or a wide comparison table fits on one sheet where Letter landscape would need two taped together — and a taped seam through the middle of a schedule is exactly where it will tear.
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.