Thanks for the reply. I am sorry probably my notation is uncommon. When I said M_{\mu\nu} I meant the representation of the Lorentz generators when acting on a Lorentz 4-vector(so antisymmetric matrices, when all indices are raised). Also the way I learned it we started at differrent reps of the Lorentz generators and then afterwards defined the fields the transformation could act on and deduced its properties - seems to me to be some sort of chicken and egg problem. However I thought that it should be possible to compute that the vector representation is (1,0)+(0,0) as this is basically the spin of the object.