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Lets say we got a space ship, interstellar freighter, MIR 2.0, death-star etc.

And for the sake of transport have it in geostationary orbit.

(Question:) How large should it be for humans on this earth like planet to see it is there, and to realize it is man made.

It is possible to see ISS when it flies by the moon ISS moon transit

but this should be without doubt visible.

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    $\begingroup$ Related: worldbuilding.stackexchange.com/questions/3768/… $\endgroup$ – Erik Nov 23 '15 at 11:56
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    $\begingroup$ Plugging in the numbers from the related answer gives you a lower bound of about 25km in diameter to be visible as a point-source; so you're looking at a big structure. $\endgroup$ – Erik Nov 23 '15 at 12:03
  • $\begingroup$ Search engines can only do so much ... thanks @Erik , flagged for closing. $\endgroup$ – Magic-Mouse Nov 23 '15 at 12:40
  • $\begingroup$ What observation technology and space travel theory knowledge does the observer have, and what timescale to determine "man-made"? Determining a satellite (even quite small) is doable as soon as you know what a satellite is; determining it's man-made (vs an aerocaptured meteorite) is easy once you see it not falling from LEO over a couple months (meaning re-boosts). $\endgroup$ – SF. Nov 23 '15 at 13:18