The simple rules of electromagnetism have no limits on the wavelength range. An enormous and continuous range of wavelengths is possible. The choice of names for different parts of the spectrum has no physical consequences.
At the very short end, it is generally believed that a consistent quantum-mechanical form of gravity will radically change the form of physics. So we guess that no wavelength below say 10-33
cm (called the Planck length) would be possible.
On the long-wavelength end, there are obvious difficulties describing any wavelengths larger than the size of the universe. Although we don't know if the universe is part of a mathematically infinite manifold, things start to get a bit awkward if you even try to describe waves extending beyond the cosmic horizon, on the order of ten billion light-years away.
The electromagnetic force is part of more unified electroweak force. The separation of electrical and weak nuclear effects breaks down on distance scales of around 10-15
cm. So on distance scales smaller than that the waves would not be standard E-M waves.
(published on 03/15/2011)