All electromagnetic waves obey the wave equation: wave speed = frequency × wavelength, or v = f × wavelength. Speed is in metres per second (m/s), frequency in hertz (Hz) and wavelength in metres (m).
In a vacuum every electromagnetic wave has the same speed, 300 000 000 m/s. The speed is fixed, so frequency and wavelength are linked: if the frequency goes up, the wavelength decreases. For example, a radio wave of frequency 100 000 000 Hz has a wavelength of 300 000 000 ÷ 100 000 000 = 3 m.
You can measure the wavelength of microwaves using chocolate in a microwave oven. Remove the turntable and heat a bar of chocolate until it melts in a few spots. The melted spots are half a wavelength apart, so measure the distance between two neighbouring spots and take twice that distance to get the wavelength. If the spots are 6 cm apart, the wavelength is 12 cm (0.12 m). Then use the oven's frequency in the wave equation to find the speed of the microwaves.