AM radio wave has a longer wavelength.
wavespeed = frequency x wavelength
Given that wavespeed is constant when travelling in air, a smaller frequency would result in a greater wavelength in order to keep the wavespeed unchanged.
Actually you can try it yourself. Find a large water surface and dip your finger into the water. When you dip your finger more frequently, you can produce wavefronts which are closer together, indicating a shorter wavelength, and vice versa.
