Hall devices having an active area of about (500 nm)2 are fabricated by focused electron-beam-induced deposition. The deposited material consists of cobalt nanoparticles in a carbonaceous matrix. The realized devices have, at room temperature, a current sensitivity of about 1 V/AT, a resistance of a few kilo-ohms, and can be biased with a maximum current of about 1 mA. The room-temperature magnetic field resolution is about 10 uT/Hz^1/2 at frequencies above 1 kHz.