You seem to have it backwards. We WANT the smallest shrapnel to fall to the atmosphere because then they'll burn up. The problem is the stuff that is way above the effects of atmospheric drag (the top of the atmosphere varies at 10-17 km) and travelling at the speed required to maintain the orbital height they're at. They might stay there indefinitely unless we learn how to make gravity guns or something. For example, some orbital speeds: 6.90 km/s @ 2000 km, 7.35 km/s @ 1000 km.