The relationship between wavelength, frequency and velocity of propagation is given by:
Velocity of Propagation = Wavelength X Frequency
Therefore, Wavelength = Velocity of Propagation/Frequency
So, if the speed (Velocity of Propagation) of sound in air is 344 m/s (actually it is strongly dependent on the temperature of air and this speed corresponds to the air-temperature of 20 degree Celsius), then
the wavelength = 344/20 m = 17.2 m, when the frequency = 20 Hz
and the same (wavelength) = 344/(20 X 10 X 10 X 10) = 0.0172 m or 17.2 mm, when the frequency = 20 kHz.