At last count I have 6 different kneeboards, of various sizes and shapes varying from smaller than A5 to bigger than A3. I pick the appropriate tool for the job, depending upon what I'm flying. This morning in a PA28 I was using an RAF issue one which is slightly narrower and longer than A5 with a pen attached and a ring binder on the top.
I'm also sorely tempted to buy one of Pooleys new helicopter ones 'cos I've seen WhirlyBird's and it impressed me.
So I don't think there's a right answer. It comes down a combination of your personal preferences, and what you fly.
G