Definitely depends on you, your preferences, and what you fly.
Personally, I always found A4 a little too big. I bought an A5 kneeboard, and designed my own plog which fits neatly onto an A5 sheet (e-mail me if you want a copy of it). My kneeboard also has plastic sleeves on the inside which I can use to store things like pages from Pooleys for the airfield I'm going to.
Using an A4 kneeboard in an aircraft with a stick would be very tricky, and in something like a Super Cub it would be practically impossible. I've done cross countries of several hundred miles in a Super Cub, and, with a little bit of planning, been able to have everything that I needed very close to hand thanks to my knee-board. (Although I have to admit that once the cross countries started getting too long to do in comfort in a Super Cub I switched to an Arrow, and it was very nice to be able to lay charts out on the passenger seat and see more than 100nm of my route at a time!)
Of course there's nothing wrong with A4 kneeboards, or folding kneeboards, if they happen to suite your style of flying better.
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