Pocket size is popular, and rightly so - it's just the size to fit in a jacket, but big enough to do something useful with.
But they can get in a mess, and Field Notes have a rather neat solution. An exactly sized box, which looks the business and will hold a few years' worth of scribbles.
Field Notes have thought this one through, too, and included dividers to separate categories; in this case mint, tested, in-use and full. But you can choose your own categories, of course. Nero's out of stock just now, but hopefully re-ordering soon.