Ideally, the size of the notebook is that of about the length of a pen (give or take ~15mm), to fit into the (mandatory) side, or cargo, pocket of your trousers (or shorts). I don't think I can wear a pair of shorts that doesn't have above 5 pockets (two front or side; two back and minimum 1 side or cargo pocket). Notepad in back pockets get crushed or bent; front pocket the pen stabs you when you're rolling about on the ground or climbing; side pocket it falls out when you are inverted or climbing. Cargo pocket for the win (zipped or buttoned). My Patagonia 'Rockcraft' pants pictured.
More pockets means more cool stuff you can bring home from your daily adventures! My wife once came out of the laundry with a 'What the fuck?!!' look on her face, having found two corks in the side cargo pockets of my shorts (along with the usual assortment of rubber bands; coins; feathers and tissues).
The yellow and orange 'extra small' notebooks are via Moleskine brand, and are small enough to fit into most pockets. Bonus points for being orange and golden yellow coloured. On thing that I don't like about the Moleskine extra smalls, is that the pages are pullout. I preferred fixed pages, personally. Moleskine are pretty good quality normally, though. Plus I think they were Hemingway's favorite; so points for that too, I suppose.
I use my notepad and pen (standard Parker metal pen.. which I have got through about 40 different airport checks, yet they didn't let me bring fingernail clippers once?? Anyway..) almost every day - to write down interesting ideas; draw; note down things I pick up in conversation, or references for books/articles that I find out in my travels.
Embrace the solar flare and EMP proof cutting edge technology of pen and notepad today! :)