GaN Schottky diode rectifiers with contact diameters 125-1100 mu m were fabricated on thick (4 mu m) epi layers. At low reverse bias voltages the leakage current was proportional to contact perimeter size while at voltages approximately half the breakdown value, the reverse current was proportional to contact area. These results suggest that surface leakage dominated at low biases, while at higher biases the main contribution was from bulk leakage. The reverse leakage currents were several orders of magnitude higher than the theoretical values, while the forward turn-on voltages were approximately a factor of two higher than the theoretical value.