Not so bad. Assuming a spherical Earth in a vacuum and a bunch of other slightly less reasonable assumptions we have:
1E+45 photons from sun per second
1.49598E+11 distance to sun in meters
6365000 earth radius in meters
2.81229E+23 surface area of solar flux at earth orbit
1.27276E+14 surface area of earth exposed to sun
4.5257E-10 ration earth area to solar flux
4.5257E+35 photons hitting earth per second
6.49E+15 one light year in meters
1.947E+25 meters in 3 billion light years
4.76367E+51 surface are of sphere 3 billion ly across
9.50046E-17 photons per m^2 at 3B ly
1.05258E+16 m^2 per photon
57883255.23 radius in meters for mirror to reflect 1 photon/second
57883.25523 radus in km (Only 10x larger than Earth!)
Then given a reasonably competent engineer to make a descent mirror with no aberration and a good telescope on a clear night away from the city, and a good bit a patience, you could watch a replay of the last 3 billion years.
1E+45 photons from sun per second
1.49598E+11 distance to sun in meters
6365000 earth radius in meters
2.81229E+23 surface area of solar flux at earth orbit
1.27276E+14 surface area of earth exposed to sun
4.5257E-10 ration earth area to solar flux
4.5257E+35 photons hitting earth per second
6.49E+15 one light year in meters
1.947E+25 meters in 3 billion light years
4.76367E+51 surface are of sphere 3 billion ly across
9.50046E-17 photons per m^2 at 3B ly
1.05258E+16 m^2 per photon
57883255.23 radius in meters for mirror to reflect 1 photon/second
57883.25523 radus in km (Only 10x larger than Earth!)
Then given a reasonably competent engineer to make a descent mirror with no aberration and a good telescope on a clear night away from the city, and a good bit a patience, you could watch a replay of the last 3 billion years.