Which electromagnetic wave has the largest wavelength? The smallest?

The electromagnetic waves that we call "radio waves" are the ones with the longest wavelengths. There's no limit to how long they can be. We actually generate, modulate, detect and USE some radio waves for communication that are more than 10 miles long, and the little bit of radiation that comes off of a power line has a wavelength of about 3,100 miles!

The electromagnetic waves that we call "gamma rays" are the ones with the shortest wavelengths. There's no limit to how short they can be. There are gamma rays with wavelengths shorter than the diameter of atoms! We have no way to generate or control gamma rays; we just stay out of their way, because they're dangerous to human tissue.