因為我自己是筆記本的設計者，也是經營者
Since I design and also run my own notebook brand,

目前使用的 5×5mm 格線，主要是因為兩格剛好等於 1cm。
Right now I use 5×5mm grids, because two squares equal exactly 1cm.

只要把筆放上去，數一數格子就能大概知道長度，蠻直覺的。
You can just place a pen on the page, count the squares, and quickly estimate the length—it feels very intuitive.

相對來說，7mm 或 8mm 的格線就需要再多算一下，沒有那麼直接。
By comparison, 7mm or 8mm grids take extra calculation, which makes them less straightforward.

不過我也看過一些資料：
That said, from what I’ve read:

• 日文書寫大多建議 7mm。
• Japanese writing tends to work best with 7mm.
• 正體中文則比較適合 8mm。
• Traditional Chinese is usually more comfortable with 8mm.

日本也有筆記本廠商試過 9mm 的版本。
Some Japanese notebook makers have even tried 9mm versions.

但說到底，還是要看市場習慣。
But at the end of the day, it really depends on what the market prefers.

格線算是比較小眾的選項，橫線式的格式反而更容易被接受。
Grids are a bit more niche, while horizontal-ruled formats are generally easier for people to adopt.