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Geographically big, if you need to see it as more than a point. 20/20 human visual acuity is around an arcminute, or 1/60 of a degree. The whole moon is about a 1/2 a degree across as seen from Earth. Below that, it's a matter at how good you are at picking reddish grey out from normal grey. Red is also scattered and absorbed pretty strongly by the atmosphere IIRC.
If it's allowed to emit light, you can go a lot smaller. I'd guess an LED array with an emitting area of 10 m^2^ would be visible from Earth as a sort of star when in shadow, without doing any actual math on it. When competing with sunlight it becomes a visual processing problem again.
If it's allowed to focus at you, too, a 1 cm aperture should be able to resolve a single kilometer on Earth, and at a kilometer of distance you can see a candle flame if it's dark enough. It's just a beefy laser pointer at that rate.