I've found that an A6 size notebook was easier to always have with me because it fit in my pocket the way a smartphone does. But I've been using an A5 since I stopped going to an office and, well, I had some A5 notebooks lying around.

I've also found that the best thing I learned to do with the notebook was to have very few things (the most important things) written down in the front and "other" things written in the back. Focusing on the things in the front until complete (or has become less important relative to some other thing) and marking them off is tremendously helpful. Folding the corner or bookmarking the most recently written on pages allows for easy navigation.

It's a bit like a stack and a heap. When a new "other" thing has my attention, I write it to the heap in the back of the notebook. The most important things (3 at most) are written to the stack in the front of the notebook and popped off (struck through) when finished or moved to the heap (writen in the back of the book because it's no long a top priority). Eventually the back and the front meet (a stack overflow) because you're not erasing anything and you need to use a new notebook.

I couldn't tell you why this is so helpful other than to say that it frees my mind to write down whatever has my attention and is important enough to remember but not important enough to prioritize now.

Other than that, I use a calendar app and Google notes for things I want in digital, synchronized form.

Also, regular exercise seems to help me stay organized. But that's probably not what you're asking about.