Most mass market paperbacks are the right size ime. For long books they tend get thick and have tiny margins though. A thinner more durable paper, if there is such a thing, might help.

There’s got to be some width/height/print size ratio that makes sense. I have a few turkey sandwich sized pocket paperbacks with teeny tiny print and it’s just like, man, I think I’d take a slightly taller book at this rate. 

SilkyNinja

It's not entirely about size, but size is part of it. I want high quality, portable, flexible books without obnoxious publisher self-promotions.