If you have to put pen to paper to sketch out your ideas, then take a look at this expandable sketchbook. While the book measures a smaller A6 size (14.8cm high and 12.5cm wide with spine), each of its 5 pages expands into a larger A3 size. Not a bad size for something that fits easily into your bag.
Also, for Design Sojourn’s 1st Birthday, they are offering free shipping for any amount of sketchbooks you purchase. (Birthday sale is over.)
Each sketchbook is $13.00 (or you can order 5 sketchbooks for $52.00).