Infant Stomach Capacity: Why Anything You can Pump is Great! - "A day old full term baby only has a 5-7 ml stomach capacity, or the size of a marble. The amount of colostrum that comes out from the mother’s breast is enough to satisfy the baby. The stomach becomes the size of a shooter marble by day 3. After a week, the stomach capacity reaches 2 oz, or the size of a ping pong ball."