What is the practical limit on very long wavelength astronomy

Theoretically, no photon with a wavelength greater than the observable universe can be detected. What's the practical limit, and why? What observations is the very low-energy part of the radio spectrum good for anyway?

I feel like this is a homework question. It sounds too well thought out for a random inquiry. That being the case, you should be researching this yourself.

I will say that no photon can be observed which has a longer wavelength than the detector. Radio telescopes require huge dishes in giant arrays to detect the signals. Obviously there is going to be a limit to how large a wavelength one can detect due to the fact that we can only build a telescope so big. However there are actually planet wide interferometers with radio telescopes. There is a system known as the VLBI which can observe wavelengths just short of a meter in length.

These observations are crucial to understanding things like Quasars and Active galactic nuclei consider they emit very strongly in the radio spectrum (primarily because of heavy redshift due to distance). Understanding these early galaxies helps us understand galaxy evolution and how the universe came to be in its present state.

each part of spectrum has its own characteristics
long wavelength requires huge diameter or separation of the receivers I do not know how low the frequency, that is how close to DC can be detected by electomagnatism