Our rumparooz newborn cover was by far the smallest. Also, newborn Nickis bamboo all in one were really small. We didn't get any thirsties size 1 until he was a little bigger but they fit similar to newborn blueberry coveralls and they fit pretty well. I preferred the coveralls but they're more pricey and I'd be wary of buying new right now anyways.
twins just home at about 4.5 to 5 pounds in Thirsties Duo size 1 over a size small/one-size birdseye flat diaper. A half size would be trimmer, but with all that cotton of the small/one-size flat in there to absorb, there are no leaks even though the cover is still so roomy. They are comfortable and leak-free.