How small can an electromagnetic wave be?

Answer 1

Arbitrarily small is an answer - some gamma rays will have a wavelength smaller than the size of an atom. Less then 10picometre = 10**(-12)m. [Where ** means 'to the power of].

Answer 2

An electromagnetic wave can theoretically have an infinitesimally small wavelength, approaching the Planck length of approximately 1.6 x 10^-35 meters. At this scale, the wave would be interacting at the quantum level and is not currently verifiable by experimental means.