What are the highest and lowest amplitude values in ECG for humans?
The QRS complex is the part of the ECG with the largest amplitude.



I scanned a dozen ECG traces across the web, and as far as I can see, the QRS complex has amplitudes of up to 4 mV in a normal ECG:


ECG trace. 1 mm corresponds to 0.1 mV on the y-axis

Amplitudes are relative peak-to-peak measures. If you are asking about absolute values then the minimal voltage versus maximal absolute voltages depend on your baseline value, i.e., the offset in your voltage potential measurement system. Further, artifacts and background noise may add signal amplitudes on top of the ECG proper.

As an absolute minimum estimate, the graphs here posted show that -1 mV would be a safe minimal voltage.