Intensitysol1 is not defined because the condition "if size(a)==2" is never true. size() returns a vector, and comparing a vector to a scalar in an if statement evaluates to true only if the condition is true for all elements of the vector, which is not the case here since the size of "a" will be 1-by-something (since "a" is a row vector). That is, size(a) is never a vector of all 2's.
You probably mean "if isequal(size(a),[1 2])", or, simpler, "if length(a)==2" or "if numel(a)==2".
Note that "if size(a)==1" works because "a" would be of size 1x1, so size(a)==[1 1], and that condition is considered true since all elements of the size are equal to 1. But here as well it is clearer to use length() or numel() instead of size().
