How large a space telescope do we need to see exo-civilizations?

[magnemoe]

1 hour ago, Exoscientist said:

 
 Thanks. I didn’t think of the uplinks. Even though it might not be possible to decrypt the signals, it might possible the spectrum is so unusual to be unlikely to be natural.

   Bob Clark

But they are weak compared to TV broadcasts and directional and moving. 
I say look for an biosphere firsts. I say this is easier than radio unless aliens try to contact us actively and will also much easier for planets to get an oxygen atmosphere than an advanced civilization. 

Then you have 

That one is seriously weird. My guess it was close to an neutron star merger some years ago so it scoped up lots of heavy elements including island of stability elements who decayed into stuff we know. 
Most likely an error so verification is always important. 

[kerbiloid]

Variants:
1. It's a destroyed alien storehouse.
Either by an accidental accident, or by a accident caused by a coming revision from the Dept. of Logistics.

2. It's a candy house trap.
Young interstellar civilizations come here, then - BANG!

[magnemoe]

7 hours ago, kerbiloid said:

Variants:
1. It's a destroyed alien storehouse.
Either by an accidental accident, or by a accident caused by a coming revision from the Dept. of Logistics.

2. It's a candy house trap.
Young interstellar civilizations come here, then - BANG!

I vote 1b, fun story, back during WW 2 US upgraded their shore batteries because of an risk of Japanese raids. You had some panics like the LA air attack who was people probably shooting at clouds or US planes. Once the AA guns started firing the rest joined in. 
Obviously the shell from the small ones had to come down somewhere, the heavier ones had timed fuses but not 20-30 mm. 
Now in 44 the threat of Japan doing raid or even shore bombardment on US west coast was laughable, but the batteries still got ammo for practice, but cleaning the guns was an pain so the simply stored most in some old magazines, for previous generations of guns. 
Then the war ended they just hide the entrance to these magazines. 
50 years later some wanting to develop the area got an surprise  in lots of cordite and old shells.