A flag would be about five hundred micro arcseconds. (about 1 meter, at around 375 Mm)
The Event Horizon Telescope has a resolution of about 20 micro arcseconds.
Therefore, EHT could resolve a flag on the surface of the moon if it were a radio source.
However, to view it optically would require around a 350 meter aperatureaperture. The largest 'single' telescope today is around 10 meters
The Cambridge Optical Aperture Synthesis Telescope (COASTCOAST) - the largest I'm aware of - can resolve to around 1000 micro arcseconds, so wouldn't be able to see a flag.