If the Arecibo radio telescope could be transported to Alpha Centauri,
scaled up in size and then pointed back towards Earth, then a telescope
with a diameter of over 33,000 kilometers would be required to
detect UHF television pictures leaked from Earth. This is equivalent
to a dish diameter just over two and a half times the diameter of the
entire Earth.
source:
http://www.computing.edu.au/~bvk/astronomy/HET608/essay/