We can use a beam plot to find the edge of a defect by using the edge of the sound beam.
If we know the width of a beam at a certain distance from the crystal, we can mark the distance across a defect from where the extreme edges of the beam touch each end of the defect and then subtract the beam width to get the defect size.

When the signal from the defect drops by 20dB from its peak, we judge that the edge of the beam is just touching the end of the defect. We can find the width of the sound beam at that range by consulting the beam plot
that we have made.

Note: The peak of the defect is normally taken as being the last peak on the screen before the probe goes off the end of the defect, not necessarily the maximum signal from a defect.