In the weird ways of narrative, I think it matters what you intend to do with the stuff you've picked up. Since it's non-Selective, basically everything you've picked up would have to move together in a big clump; you couldn't pick up a pile of 50,000 1lb rocks and fire them machine gun style at targets with that effect (although that could be a perfectly good AE or power stunt). Also, if you threw what you had lifted at a target, it couldn't inflict more damage that the MO rank, whether you allowed lifting several objects around your maximum weight or not; on the other hand, picking up several heavy objects and attacking an area would be reasonable, as long as it was within PL.
As long as it was within those limits, I think the collective weight of the Area MO shouldn't really matter. So for example, a magnetic-based villain with Area MO 6 could easily lift a whole bunch of police cruisers up into the air and throw them as an area attack at a pack of heroes. EDIT - And this would behave essentially identically to picking up a big pile of scrap metal, or a billion ball bearings.
As for "where to stop" I think making a blanket ruling for a naked effect like this just doesn't make sense, so it's really up to you and the GM as to how all-encompassing the area effect is given its descriptor, but it should be consistent every time the power is used. If it's a big spherical magnetic field, it should take every metal piece with it. If it takes soft ground with it (soil, sand, gravel, etc), it should do so every time unless the ground is too hard and tough. If it generally leaves smaller objects behind (say anything smaller than rank -5, or 6 inches) and only picks up big things like some kind of super-sieve, it should do that every time.