Telescope resolution is calculated with "Dawes's limit" equation.
a = 116/D
where a is the smallest resolvable angle (in seconds of arc) and D is the "aperture" (usable diameter of the lens or main mirror of a telescope) in millimetres.
116 is the constant for visible light.
For longer wavelengths (like infra red) the resolution gets worst = you cannot see small details as easily. For shorter wavelengths (like X-rays) the resolution would be better except that... X-rays go right though mirrors without getting reflected.
A telescope on Earth (or in a low Earth orbit) is roughly at 380,000 km from the Moon, on average. Because the Moon's orbit is an ellipse, there are time when it is closer.
Let's be very generous and say that the closest distance is only 300,000 km (it never gets that close, but let's play along).
The base of the lander measures 4.27 metres across. However, if we include the extended landing pads, we could get a total span of about 9 metres (almost 30 feet).
I'll be generous and use 9 metres (easier numbers), even though the pads are probably covered with lunar dust after the blast-off.
Keep in mind that the flag is much, much smaller than the base of the landing module.
At 300,000 km (= 300,000,000 metres), a nin-metre object subtends and angle of:
9/300,000,000 radians = 0.00000003 radians.
There are 206,264.8" in one radian.
206,264.8 * 0.00000003 = 0.00619"
Or, as the kids now like to say: 6.19 mas (milli-arc-seconds)
Back to Dawes's limit:
a = 116/D
If we need to find D:
D = 116/a = 116 / 0.00619 = 18,746 mm = 18.75 metres (61.5 feet = 738 inches).
We'd need a telescope (or a camera lens) with a diameter of 18.75 metres.
The Hubble Space Telescope has an aperture of "only" 2.4 m (94 inches).
Telescopes on Earth are affected by the atmospheric turbulence and they are limited to resolutions of (at best, with adaptive optics) 0.01"
To be seen by an 18 m telescope on Earth's surface, a lunar object would have to be at least 15 metres across (under perfect viewing conditions).
In any event, the largest telescope on Earth is less than 11 metres in aperture.
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Sure, we (humans) have satellites around the Moon. They are much closer, but they are generally equipped with much smaller cameras (aperture counted in inches). So they too are not equipped to find objects that small (I'm still talking of the entire lander base, never mind the flag).
I guess we'll have to wait a few more years.