I e-mailed small-beginnings.com because they have tiny NICU diapers that fit micro preemie reborns.
The Cuddle Buns come in sizes 21C-under 2 lbs: 21A 2-4 lbs and 21B 4-6 lbs.
The Beary Small come in 22C -under 1.5 lbs: 22A 1.5-2.5 lbs and 22B 2-4 lbs.