It depends what purpose you are using the sketchbook for. An A3 sketchbook is good for planning, as you can fit quite a few illustrations on one page and it is quite useful for storyboarding. If you are working on location in a busy place and you want to be discreet i.e. You don't want people leaning over you, then an A4 or A5 sketchbook may be useful. You may be limited if you use this size on location, but it will help you to avoid distractions from the public. If you are planning to sketch a landscape in a place that isn't as busy, then an A3 or A2 sketchbook would be useful. Alternatively, you could use a board with a single sheet of paper attached to it with masking tape for sketching if you want to avoid using a whole sketchbook.