The size of diaper you will require will depend on the kind of diaper you want to use.

One Size Diapers
These diapers will generally adjust to fit from newborns to potty training.
FuzziBunz® One Size Diapers fit from 7-35lbs
Sweet Pea One Size Diapers fit from 8-35lbs
Kawaii One Size Diapers fit from 8-36lbs

Prefold Diapers
Prefold diapers like the Bummis brand that we carry come in:
Preemie (4-9lbs), Infant (7-20lbs) and Baby (15-40lbs) sizes and must be accompanied by a cover.

FuzziBunz® Perfect Size Diapers
For those considering the FuzziBunz® Perfect Size, use the following chart for reference: