Manuscript paper, A5
Blank staves on A5, small enough to carry and large enough to write on properly.
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A sketchbook, not a score
A5 will not hold a full score and is not trying to. It holds a melodic idea, a chord sequence, a bass line you thought of on a train. The value is that it fits in a bag or a coat pocket, which means it is present at the moment an idea arrives. Ideas that wait for you to get home mostly do not survive the journey.
Staff size matters more on a small page
The 2.5 mm figure is the gap between staff lines, so a full staff is four times that. On A5 you get roughly eight staves at this size, which is enough for a sixteen bar sketch. Going smaller to fit more is a false economy: notation you cannot read back at a glance is notation you will have to decipher, and the whole point of a sketch is speed.
Print two up on A4
Most home printers do not feed A5, but two A5 sheets fit on one A4 page and the driver will usually do that for you under a two pages per sheet option. Print, trim down the middle, and staple a stack. Just make sure the driver is not also scaling the page, since the two pages per sheet option and the scale setting are separate controls and the calibration sheet will tell you if both fired.
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.