Is there a cap to EM wavelengths I was just wondering, if there is a cap to the min and max of EM waves.
like whats the smallest possible wavelength? the width of an electron? smaller? What kind of wave would that be? 
And what about the other end? If I'm not wrong radio waves are really long but what would we get with a wavelength of like 1 km?
 A: Well, 1km wavelength is not unusual. It's at the top of the Low Frequency (LF) band:
https://en.wikipedia.org/wiki/Low_frequency 
and it is surpassed in length by the very-, ultra-, super-, and extremely-low bands (VLF, ULF, SLF, and ELF, respectively). ELF goes down to 3Hz (100,000 km).
Of course, there are lower frequencies, with 0Hz corresponding to a static field.
At higher frequencies, we generally consider single photons. Accelerators can produce Bremsstrahlung up to their beam energy. 
Higher energy photons come in the form of cosmic rays, which are detected via the Cherenkov shower detectors. Very-high-energy gamma rays (VHEGR) ranging from 100 GeV to 100 TeV have been detected. Recall that:
$$ \hbar c \approx 200 \,{\mathrm{MeV\cdot fm}} $$
so VHEGRs' wavelengths can be a short as
$$ 2\times 10^{-6}\ \mathrm{fm} = 2\times 10^{-21}\ {\mathrm m}$$
Higher energy photons are called Ultra-High Energy Gamma Rays (UHEGR), and are the subject of cosmic-ray research. At these extreme energies pair-production can occur in a static magnetic field (e.g., the Earth's field), so that propagation becomes non-trivial. (See: Landau–Pomeranchuk–Migdal effect).
There is also the Greisen–Zatsepin–Kuzmin limit, which should remind you that wavelength is not a Lorentz scalar. In the reference frame of an Ultra High Energy Cosmic Ray, some of the cosmic microwave background radiation is extremely short wavelength photons.
A: Theoretically:
There is no maximal wavelength. One could say that the observable universe is the max, but then we could say that the whole universe is infinite.
There is no minimal wavelength. One could say that the Planck scale is the minimal that makes sense, but that is a misconception.
Even at wavelengths smaller then the Planck length, we would be nowhere near the energy of the universe.
In practice:
The shortest ever detected is 16Tev gamma ray, which means 1 billionth of a nanometer.
The longestwavelength is 40000km, which is for the Schuman resonance, for 7.83Hz. It is caused by 1000 lightnings per second, and is a standing wave.
