I cannot tell you how big an area the Fidelius Charm covers. For all I know, MMA covers ten square miles, far more than the old brick building that you see. All that I can say for sure is that the western side of MMA’s Fidelius Charm coincides with the western wall of the glamoured brick building, and that the only wardstone that for sure is where it seems to be is the wardstone that is by the south-west corner of the illusionary brick building. Destroy this wardstone and you create a gap in MMA’s wards-protection near the wardstone.