Electromagnetic waves can span a wide range of wavelengths, from extremely short to extremely long. Theoretically, there is no upper limit on the size of electromagnetic wavelengths. However, in practice, the largest known wavelengths are associated with extremely low-frequency radio waves and are limited by the size of the universe.
The largest wavelength observed so far is the cosmic microwave background radiation (CMB), which has a wavelength of about 1.9 millimeters. This radiation is believed to be the remnants of the Big Bang and fills the entire observable universe.
If we consider even larger wavelengths, we start to enter the realm of hypothetical concepts such as cosmic strings and other cosmic structures. These structures, if they exist, could potentially generate even longer wavelengths. However, such phenomena are purely speculative at this point, and our current understanding of physics does not provide a clear limit on the size of electromagnetic waves.
It's worth noting that as wavelengths increase, the corresponding frequencies decrease, resulting in lower energy levels. Very long-wavelength electromagnetic waves, such as those associated with radio waves, have low energy and are generally not harmful to living organisms.