I tend to have more strips than chunks of leftover batting, but I can make use of pretty much any strips 3-4" wide or bigger. I'd be inclined to go a bit larger if they're chunks - maybe 4-5" in that case. It all depends on how much effort I'm prepared to put in. With these smaller pieces, I do try to pair them up with slightly larger ones (I've never tried to make a "string batting," for example) to camouflage the surgery scars and I make battings that are smaller overall, mostly for MOW placemats. I've done a few of these lately and even when I know exactly where the joins are, I haven't been able to feel them through the fabric. I imagine the same would hold true if you were going to make QAYG blocks (not blocks that are going to have exposed edges like the rag quilts you mention).