Choosing a brand of diapers can be challenging in itself but choosing the correct diaper size can be more challenging. There are 7 to 8 different size diapers. The sizes include Premies, Newborn, Size 1, Size 2, Size 3, Size 4, Size 5 and Size 6. Each of these sizes presents different weights for the baby. For example, Size 3 diapers are made for babies who weigh 16 to 28 pounds.