Manuscript paper, A3
Blank staves on A3, twice the area of A4, which is what you need once you are writing more than two staves at a time.
Opens with these settings already loaded. Change anything you like, then download as PDF, PNG or SVG.
Why score work outgrows A4
A single instrument fits comfortably on A4. A string quartet needs four staves braced together per system, a wind quintet five, and a full score considerably more. On A4 you get one or two systems per page and spend the whole piece turning pages; on A3 the same music sits in half as many pages, and more importantly you can see the vertical alignment between parts, which is the entire point of a score.
Stave size and legibility at the desk
The 2.5 mm figure is the gap between adjacent stave lines, which is the standard measure of stave size, a full stave is four of those, so 10 mm. That is comfortable for desk work with a fine pen. If the copy is going on a music stand and read at distance, or if the player has any difficulty with small print, go up to 3 or 4 mm before you print; the large-stave sheet is set up for that.
Printing A3 without an A3 printer
Most home printers stop at A4, but A3 is standard at print shops and in most offices, and it costs very little per sheet. If you cannot get A3 printed, the honest fallback is landscape A4 with fewer staves rather than shrinking A3 down, reducing an A3 sheet to A4 takes the staves to about 71% of their intended size, which is small enough to be genuinely hard to write on accurately.
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.