The Moon's diameter is approximately 3500 km. The Moon is about 400,000 km from Earth on average, giving it an angular size of about 30 arcminutes in the sky as seen from Earth. There are 60 arcseconds per arcminute, so the Moon is about 1800 arcseconds across. 1800 arcseconds = 3500 km, 1 km = .51 arcseconds. An average flag is about a meter wide, let's say. So the flag will make up about 0.00015 arcseconds in angular size.

My personal telescope, with a 3.5"/88.9 mm aperture (0.0889 m), has a resolution of about 6.19e-6 radians, or 1.28 arcseconds. That means the smallest point of light distinguishable in my telescope (which is an average astronomical telescope) is around 8,500 times larger in angular size than the flag at the landing site. The lander, being maybe nine times larger in angular size, would still be about 950 times too small to make out with an average telescope.

To be able to make out the flag at all, you'd need a telescope with an aperture of 272 meters, or 894 feet. To make out the lander, you'd need a telescope with an aperture of around 99 feet. However, keep in mind that this is just to technically be able to make it out as a single pixel. To make them sizeable enough to know what you're looking at, you'd need probably 250 square pixels, so the aperture size would need to be another 50 times larger, making for a minimum of about a one-mile aperture. So the opening on your optical telescope would need to be one mile in diameter.

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