The average stomach holds 1 to 1.5 liters. Initially, the human stomach is about as big as a closed fist, but it can expand to a much larger size. It will shrink back to normal after a time.
It is a muscular organ and size varies according to the stature of the body.Normally it is the size of a hand stretching full when filled with food and returns to narmal after digestion.Here I assume the intestinal tract as stomach.Too big a size is dangerous.
I have always heard that the normal size of the human stomach was comparible to the size of the persons heart or fist. This could be different due to the expansion of the stomach when a person overeats.