This is a great article with a chart showing the size of a newborn's tummy days 1-10. This is why colostrum, minimal though it is, is the perfect food for a newborn. (in addition to the great immunological benefits and laxative effect to clear out meconium, etc,)
Day 1 it is the size of a shooter marble and does NOT stretch. Feeding more than this this amount causes spit up.
Day 3 it is the size of a ping pong ball.
Day 10 it is the size of an extra large chicken egg
adult - softball size