When to switch diaper size newborn to s size?

It depends very much on the size (weight, build) of baby during birth. For baby above 4kg at birth even can start to s size diaper. Normally after baby turns 1 month old. Some diapers' size such as Merries, the cutting will be slightly bigger and can use until baby is much older. When you feel the baby's thighs are tight with the diaper or urine/poop leaking, it means it's time to switch to diaper which is one sized bigger.