It was demonstrated that with increasing probe length the electron saturation current becomes more independent of the probe potential. This provides higher accuracy of measurements of the electron density and plasma potential in a strongly magnetized plasma (B>0.1 T). Long probes oriented parallel to the magnetic field yield better spatial resolution in the perpendicular direction. Parallel probes, however, are very sensitive to alignment errors. To avoid this problem, a cylindrical probe with a side surface coated by a thin insulating layer is proposed.