Notebook sizes - Standard paper sizes
The most easily understandable and consistent notebook sizing are those that use 'standard' paper sizes. These most commonly include the A series and B series, these come from ISO print standards, which means they are consistent.
A4 is the most common paper size — it's the size we use in photocopiers, for example. It's also a common notebook size. A5 is also extremely common in notebooks.
The B series of standard sizes is also quite common, especially in Japanese and Korean notebooks.
TIP: The handy way to remember the relationship of any of the A sizes, is that each size is half of the smaller number. So A4 is half of A3, A5 is half of A4, A6 is half of A5 and A7 is half of A6.