The Apollo missions left six nylon flags on the moon, which are probably intact today. They’re about 1.5 meters long by 0.9 meters tall, meaning that the stripes are 69 mm tall. The first flag was blown over during the takeoff of the lunar lander. The LRO in 2012 showed that the three flags it imaged remain intact and erect.
What would it take to get 500-mm resolution from a terrestrial telescope? Suppose we can use 300-nm blue light. The Airy limit is sin θ = 1.220λ/d for a circular aperture, and the moon is 384 megameters away, so our angular resolution needs to be about 1.3 nanoradians. This gives the telescope aperture diameter d = 280 m.
The largest single-aperture optical telescope currently is the Gran Telescopio Canarias, which is 10.4 meters in diameter.
In the last 30 years, substantial progress has been made on long-baseline optical interferometry, and there is one such telescope in the US with a sufficient baseline: the Navy Prototype Optical Interferometer with its 437-meter baseline, which has a resolution of a few milliarcseconds. This is unfortunately not quite good enough: 1.3 nanoradians is about 0.27 milliarcseconds. The Very Large Telescope in the Atacama, the first telescope to image an etrasolar planet, has comparable resolution, down to a single milliarcsecond. But these telescopes are designed to image bright objects such as stars, not dim objects such as points on the moon.