One medium tomato (1/3 pound) yields enough for a salad for one person, a large tomato (3/4 to 1 pound) yields enough for two people. So scale the recipe up or down accordingly. If you are serving 4 people, you'll need about 4 medium-sized tomatoes or 2 large tomatoes. (These proportions yield a generously portioned salad.) For a completely different flavor, you can substitute the mozzarella with thin slices of Parmesan. Feel free to omit the balsamic vinegar if you want highlight more of the flavor of the tomatoes.