Hi!
Funny thing is, that I never use a kneeboard the way I should too. I just have one and put my charts and papers in it, but I never put it round my knee, because I hate to have my legs being "blocked" by something, especially when I have to do much leg-work (cross-wind-landings etc., single-engine-failure...).
But I do think, that anything bigger than A4 is not usable in small aircraft.
Bye,
Bernd