For the start, begin with baby newborn bottles with 4 or 5 ounces, as they can hold the perfect small amounts of formula or breast milk for the newborns to drink in one sitting. Gradually, by the 4th month, you can shift to 8 or even 9 ounces of newborn bottles that can hold a good amount of milk for the little one. Similarly, as the baby grows, you can shift to higher ounces or as the baby increases their appetite.