Seriously, you want to know how to choose a sketchbook. It depends on what you want. Sketchbooks come in all shapes and sizes.
What exactly is it that you want? You need to ask yourself what will your sketchbook be for. Will you put completed drawings in it? Will it be just for putting down rough sketches or ideas on the fly? What media will you be using?
There’s no “one size fits all” sketchbook out there. They literally come in many shapes and sizes. There are small ones like that one you see above (my red Moleskine pocket sketchbook). There are also more medium sized ones as well as huge books. I also happen to own a 11×14 inch hardcover sketchbook.
It all depends on your needs when you choose a sketchbook. If you want something to carry around with you to scribble ideas down in, a smaller one may be your choice. If you want something to put down detailed drawings in or even sketches intended for finished works (like a landscape or still life), a more medium size would be better. Or you could go for the bigger sizes.
What will you draw with? Most sketchbooks will take dry media like pencil and graphite, charcoal, crayon, chalk, and pastel. If you plan to use pen-and-ink or watercolors, you’ll need a stronger paper.
In fact, I myself will be using a watercolor sketchbook for making drawings and scribbles with the medium of pen-and-ink.
Do yourself a favor however. Do NOT get the cheapest sketchbook out there. Invest in something of good quality. A hardcover sketchbook is more durable than the paperback kind.
All in all, shop around, see what’s out there, and consider your needs and wants. Choosing a sketchbook depends on who you are and what you want out of drawing.