In this paper, the main objective is to optimize permanent magnet synchronous generators for offshore direct drive wind turbine, examining the best choice of magnet grades, BHmax, and working temperature. A surface-mounted Nd-Fe-B generator is designed electromagnetically and structurally and optimized for different rated powers of 6, 8, and 10 MW. The results show that the cost of energy decreases as the wind turbine's rated power increases. Further optimizations were carried out using different neodymium magnet grades, and it was found that the higher magnet grades produce a lower cost of energy. In addition, steps were taken to estimate the effect of magnet temperature. A detailed thermal model is used to calculate the cooling airflow requirements to bring the magnet operating temperature from 120 to 80 degrees C. Allowing the use of cheaper temperature grades of magnets, the additional cooling reduces winding losses and improves the effective BHmax of the magnets.