I often think about this. But what is the best page size for notebooks?
For a planner, like a Hobonichi, I think the ideal size for me is a A5. Because then you can do a multi-column setup and it isn’t too cramped to do month or week spreads.
For portable notebooks, I don’t have much of an opinion, I mostly use Travelers notebooks and their passport size. Not because they are that much better than anything else, but I like the system, they have various useful formats, including dot grid and better paper quality than most.
For writing, bullet journaling, lists and such, I mostly use A5, but based on how much of the pages I fill something like a “A5 slim” would have been better. Not sure of A5 slim is a real thing or just a fake standard JetPens put on the Travelers refills.