Many parents start their baby with a 2-, 4-, or 5-ounce bottle and then work up to 8-, 9-, 10-, or 11-ounce size as their baby grows. If your baby seems upset about the bottle, try swapping out the nipple for one with a faster or slower flow speed to see if that helps. In general, younger babies need a slow flow so they don't get a face full of milk; older babies can handle a faster flow and might act hangry if they can't gulp as much as they want.