Answer: the wavelength decreases
Step-by-step explanation:
As long as frequency and wave speed is consistent, wavelength remains constant.
Extreme amplitude changes can affect the wavelength by changing the wave speed. Think of waves in a string, for instance. The string is stretched when the amplitude is very high; as a result, the average tension is higher, increasing the wave speed and shortening the wavelength.
Without necessarily altering the fundamental frequency, adjusting the amplitude also adds additional frequencies.