I'm pretty sure validateattributes can't do what you want.
If in fact A is required to be a column vector, you can use
validateattributes(A, {'numeric'}, {'column'}); % change class as appropriate
but if it's OK for A to be empty, or if it's possible for A to have 3 or more dimensions, then you need to write out the check in the conventional way:
if size(A,2) >= 2
error(...);
end
If you are using writing a checking function for inputParser, the answer is that it's possible to do what you want but a little more complex - please say if you need that.
