Each sensor increases the range which you ship can scan with the sensor console by about 6km. The sensor station has about 6km range as well.
100km-200km range will allow you to scan any planet and its moons while orbiting the planet.
2-3 million km range will allow you to scan every planet and moon in the system while orbiting the star. (Or standing next to a wormhole, as they are pretty close to the star)