Radio wave
The smallest radio waves are called microwaves.  Shortwaves are not quite so small.  There are also medium and long waves. Antennas designed to send and receive radio waves are usually similar in size to the wavelength they are to use. Many radio antennas (like those on cars) are made long because they receive signals of FM radio (a few meters, several feet) or AM radio (hundreds of meters, about a thousand feet).