For diapers:

Size 1 ranges from 2 to 5 kg
Size 2 from 3 to 6 kg
Size 3 from 4 to 9 kg
Size 4 from 7 to 18 kg
Size 5 from 11 to 25 kg

Choose the size of diapers corresponding to your baby's weight. If you hesitate between two sizes, always choose the larger size, this way your baby will always be comfortable and the risk of leakage will be reduced.