I think it is fair to say that I have used everything between the Field Notes pocket size and A4 over the years.
When I was a student I loved the A4 size, because I wrote notes all day either in lectures or while reading.
These days, I use two sizes: A5 and pocket sized.
All the different sizes have their own strengths and weaknesses. And the only way to figure out what works for you is to experiment. I did that for close to 10 years and I ended up with the two things that works the best for me.
Both of my notebook sizes of choice has emphasis on portability. This is because I do end up bringing them places a lot. All my A5 notebooks are hardcover. This is because it makes them more versatile and easier to write on places without a desk. I have three of them: one lined for writing, a dot grid for notes and lists and a 5 year journal.
And I also have a Field Notes in a Bellroy cover for when I need something very light and portable. This is what I typically bring when I’m hiking or either have a small bag or no bag at all.
I have tested out both hardcover pocket notebooks and softcover ones, and I ended up on softcover. They are less practical because it is much harder to write on without a desk or table. However the pocket sized format is very flawed and the key attribute I look for in them is being portable. And softcover notebooks are thinner and more portable. Therefore I consider softcover to be the better option.