Khalad is measuring the amplitude of a wave. What can be known about this wave?

The answer is the wavelength.

If Khalad is measuring the amplitude of the wave, therefore, he might also learn the length of the wave. The amplitude of the wave refers to th distance from a rest position to the crest position which distance is half vertically from a trough to a crest. By looking at the amplitude, Khalad doesn't only see the distance but also the length.