When b=0, z is a real number and the size can be compared;

When b is not zero, z is imaginary, (when a=0, it is pure imaginary), and the sizes cannot be compared.

In mathematics, the definition of size is that on the (real) axis, the right one is bigger than the left one. However, the representation of complex numbers needs to introduce imaginary axis, which is expressed on the plane, which does not conform to the definition of big and small, and it seems meaningless to define the size of complex numbers.