In a side by side comparison to the Imse Vimse Premie diaper covers they are about the same size--and Imse Vimse covers say they start at 4 lbs. I don't know. I bet they would work for some babies it would just depend on how they were shaped.
I had a 6 pound 6 OZ that dropped to 5 lbs 4 oz. I loved preemie prefolds with prowraps newborn OR kissaluvs or muttaqin newborns with dappi pullons. I also liked fuzzibunz petite - those fit very nicely also.
I didn't try bummis newborn but you can get prowrap seconds for very little and dappis are still one of my favorite covers.
Newborns should be fine. Green Earth runs big, though, so you might want preemies if your babe is a skinny minnie. My girls can still easily get into a Green Earth preemie at 11 pounds but the Prorap preemie is just a bit too small.