We can get a rough estimate from the transmissions detected from the Pioneer 10 spacecraft:

EARTH STRAINS TO HEAR PIONEER 10 SOME 7 BILLION MILES AWAY.
http://www.xs4all.nl/~carlkop/pioneer.html

The transmitter is 7.5 watts and the transmissions were detected using NASA's 70 meter radio telescopes in its Deep Space Network. The furthest detections occurred when Pioneer 10 was more than 10 billion kilometers away (it recently stopped transmitting or the signal strength dropped too low.)
Television broadcast stations send out transmissions at the megawatt scale. So let's say an alien broadcast is a million times stronger than the Pioneer 10 signal at transmission. The strength of a signal drops by the square of the distance. So we could detect such a signal not a million times further away than the Pioneer 10 signal, but only a thousand times further away. So this signal could be detected 10 trillion kilometers away. This is the distance of 1 light-year!
If you wanted to be able to detect signals out to 10 ly, which includes several stars, the signal strength would drop by a factor of 100, so the collecting area would have to be 100 times as large, which means the diameter of your telescope has to be 10 times as big. This gives it a diameter of 700 meters. This is only twice as wide as the current largest telescope the Arecibo radio telescope.
This is only a rough guess because we know the frequency Pioneer 10 is transmitting on and we know what the signal is supposed to look like, which is not the case with a supposed alien signal.

This link shows stars to within 30 ly:

The Closest Stars
http://www.dudeman.net/spacedog/const/close.shtml

One of these at 10.5 ly away is Epsilon Eridani. It was recently shown to have an orbiting planet:

Epsilon Eridani.
http://www.solstation.com/stars/eps-erid.htm

Bob Clark