Graph paper, A3
Five-millimetre squares across a full A3 sheet, with a heavier line every centimetre. Twice the drawing area of A4, and the smallest ISO sheet used for drawings with more than one view on them.
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Twice the area changes what fits
A3 is exactly two A4 sheets side by side, so a 5 mm grid fills it with roughly 56 by 80 squares. That is enough for a complete circuit schematic, a site plan, or a graph with the resolution to show individual data points rather than a smoothed trend. Work that needed splitting across two A4 pages generally sits on one A3 sheet without compromise.
Printing A3 without an A3 printer
Most home printers stop at A4, and two approaches work. Print the PDF at 100% across two A4 sheets using your driver's poster or tile setting and butt the halves together — the grid lines meet exactly if nothing has been scaled. Or take the file to a copy shop, where A3 costs very little. What does not work is printing A3 shrunk to fit A4: the squares become 3.5 mm and nothing on the sheet measures.
Line weight scales with viewing distance
A sheet this size is usually pinned up or looked at from further back than A4. Lines that read comfortably in the hand can disappear at arm's length. Nudge the weight up one step, and consider darkening the centimetre majors specifically — that way the structure carries across a room while the fine grid stays quiet when you lean in to draw on it.
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.