Actually a lot bigger than that. If the flag is about 2m across (I think it's only 4ft but 2m will do), then that will subtend an angle at the earth that is a tiny fraction of an arcsecond, (about 0.00033). The Dawes limit for a telescope, or the smallest angular distance that can be resolved, is 116/D where D is the diameter of the objective in mm. So doing a bit of algebra and solving for D gives an objective size of 386m. The biggest one on Earth is something like 10m so nothing we currently have on Earth could image the flag.
However, even if we did have a c400m telescope, the atmosphere gets in the way, the effect of which is to bounce the image around so that it sort of blurs and means that there are limits to the size of object that can be viewed. The best 'seeing' on Earth rarely gets better than 0.5 arcseconds which means that anything smaller than that, e.g. our flag, will be a blurry mess and can't be resolved. It can be compensated for with some special adaptive optics but the upshot of all this is that, no, you can't see the flag from Earth through a telescope.