|I used OS and S Sandys (with S AFs) from 6.5 lbs without leaks on my scrawnie little preemie. They look big and don't fit terribly snugly, but they don't need to. I was so sick of leaks with disposables that I figured it was worth trying the OS that early, and it worked great. That was with my second, so the bulk didn't bother me. For most babies, OS work just fine for a newborn, but it's the bulk that causes some people to prefer a trimmer option, like the XS Sandys or the prefolds/snappi option. I'm not criticizing that choice at all, just saying it's usually about preference, not performance.
If you look into prefolds, make sure you look at quality ones (think they're called diaper service quality often?), probably only available online or a diaper store--not the Gerber prefolds at Target/Walmart, since those are great burp cloths, but from what I've gathered, not the greatest diapers.