I played with different inductor sizes but 7mm was very hard to achieve. But my main concern is not the distance. It's the stability of the sensor and the response time. (I believe the stability problem comes from the fact that I use a bigger inductor that requires more samples) If I keep increasing the size of the inductor then the OSC frequency will drop to the point when I'll have to increase the number of samples the PIC takes before it can accurately detect metal presence, which will increase the response time. But even now, the sensor is too slow for my needs. The response time of the sensor is 18ms (watchdog timer) * 2 (pre-scale factor they use) = 36 ms, which is way above 1ms that I'm aiming for.