audio change object size

It all depends on what certain part of the audio you want to base the scaling - you could for example check the dB value:

 audio.GetOutputData(samples, 0); // fill array with samples
 float sum = 0f;
 for (int i=0; i < qSamples; i++){
 sum += samples[i]*samples[i]; // sum squared samples
 }
 rmsValue = Mathf.Sqrt(sum/qSamples); // rms = square root of average
 dbValue = 20*Mathf.Log10(rmsValue/refValue); // calculate dB
 if (dbValue < -160) dbValue = -160; // clamp it to -160dB min
 Debug.Log(dbValue);

Now, based on this, give a proper relation to the scale of an object.

Something like gameObject.transform.localScale = new Vector3(dbValue, dbValue, dbValue);