We investigated the performance of Hall sensors with different Co-C ratios, deposited directly in nanostructured form, using Co-2(CO)(8) gas molecules, by focused-electron or ion-beam-induced deposition. Due to the enhanced intergrain scattering in these granular wires, the extraordinary Hall effect can be increased by two orders of magnitude with respect to pure Co, up to a magnetic field sensitivity of 1 Omega T-1. We show that the best magnetic field resolution at room temperature is obtained for Co ratios between 60% and 70% and is better than 1 mu T Hz(-1/2). For an active area of the sensor of 200 x 200 nm(2), the room temperature magnetic flux resolution is phi(min) = 2 x 10(phi 0)(-5) in the thermal noise frequency range, i.e. above 100 kHz.