Most babies will continue to wear size 1 diapers until 7 months of age and then by 7 months of age or 8 months of age the 7 or 8 month old baby will be ready to wear size 2 diapers.
Size 1 diapers will usually fit up to 14 lbs and then size 2 diapers fit up to 18 lbs.
Some babies are smaller or larger than others so the baby may need the size 2 diaper sooner than another baby who still fits in size 1 diapers.
My babies though needed size 2 diapers by age 7 months old and then by 1 year old they needed size 3 diapers and by age 2 they needed size 4 or 5 diapers and by age 3 they needed size 5 diapers and one needed size 6 diapers by age 3.
My kids were potty trained before age 4 so we didn't need larger size diapers.