Genko yoshi, B5
Japanese manuscript paper at B5, the size Japanese school stationery is actually built around.
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B5 is the Japanese school size
Japanese notebooks and exercise books are overwhelmingly B5, so genko yoshi at B5 files with everything else in the bag. Note that Japanese B5 is the JIS standard at 182 by 257 mm, slightly larger than the ISO B5 of 176 by 250 mm used here. The difference is about 6 mm across and 7 mm down, which is invisible for loose practice sheets and worth knowing if you are printing inserts for a Japanese notebook.
Squares count, not words
Standard genko yoshi holds 400 characters, twenty columns of twenty, and Japanese writing length is stated in sheets rather than word counts. An assignment described as 30 sheets means 12,000 characters. Schools, literary prizes and publishers all specify length this way, which is the main practical reason to compose on ruled paper rather than in a word processor.
The gutter beside each column is not margin
The narrow channel running alongside each column is for furigana, the small kana printed beside a kanji to show its reading. Leaving it out is the most common error in improvised versions of this paper. Teachers and editors expect to find readings there, particularly for names and for kanji outside the common list. The A4 version is the same ruling on a page any printer will take.
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.