1550 Nm on a 12 mm shaft is a huge torque. The stress in the shaft will be enough to break it.
Pick the material for your shear pin. Something softer than the shaft. Find the shear strength from a table or experiment.
Calculate the shear force by dividing your torque by the 6 mm radius. This force acts on the cross sectional area of the pin in two locations. (Each side of the shaft)
Divide the force by the shear strength to get the shear area which you expect to fail. Divide the area by two and determine the diameter from that value.