Size 0 baby refers to a baby clothing size that is typically labeled for babies between 6 to 12 months old, with a weight of 10 kg and a height of 76 cm. However, it is important to note that baby clothes sizes can vary between brands and that sizes represent an average range, with the size listed being the upper limit of that range. It is recommended to size up when in doubt, as babies grow fast and clothes can be rolled up if they are too big.