Radio Waves

Generally, if a radio wave is 6 meters (18 feet) how long or tall should the antennas be?

Usually half or a quarter of the wavelength. This being said, half the length of an antenna for a 6 meter or 18 feet radio would be 3 meters or 9.5 feet. A quarter of the wavelength is 1.5 meters or 4.9 feet long or tall.

What is “gain” referring to?  

Gain is a technical measurement that comes down to the amount that can boost the signal. If there is an antenna and it is on, the signal will be clearer and you will gain more of the signal than with it turned off.