Ultimately, no one expects to find the one greatest notebook size, regardless of how much research one might delve into beforehand. It's a trial and error process that involves trying out one particular size, living in it for a stretch of time, and seeing how it works with your flow. If the size doesn't fit your creative habits (i.e. it's too big to carry in your pants pocket, too small for your large, loopy handwriting, or too cumbersome to carry in your bag. Thankfully, there are so many options available. If one size doesn't work, you can try a larger/smaller size to better suit your needs.
Also, who expects to only use one notebook all day long? Try purchasing an assortment of sizes - let's say an A7 passport, a B5, and an A4 - and set them to use for specific tasks. You could carry the A7 around with you during walks and commuting. The B5 could be at your work desk to plan your day. The A4 could be for drawing, calligraphy practice, and other artistic pursuits.
However you decide to use your notebooks, we hope that this guide will help you find the size that fits comfortably and allows you to be more creative and productive than ever before.