How big a lunar radio telescope do we need to detect radio loud aliens?

A problem with the SETI search is it looks for a specific radio frequency and even worse they have to be directly point at us to be detectable.

We can’t from Earth just try detecting normal radio signals like we put out with radio, television, cell phones, etc. because from other planets it would be completely drowned out by our own transmissions.

There is a plan now to put a radio telescope on the far side of the Moon to get a highly sensitive radio telescope not suffering from interference from Earth transmissions. How large would it need to be to detect radio signals like we put out, to, say, 50 lightyears?