How do I know the "serving size" of a food?

The Substitutions List in our FAQ describes the serving size for most foods. Serving sizes vary. For example, a "serving" of fruit could be one medium-sized piece of fruit, 4 ounces of unsweetened fruit juice, or 2 tablespoons of raisins. A "serving" of bread is one (1-ounce) slice of bread, 1/2 cup of cooked cereal or 1/2 cup of cooked dried beans.  More information can be found here