Serving size is the amount of food recommended for one person to consume in one sitting. Sometimes this amount is only a portion of an item. Nutrition facts are reported by the serving size.
For example, a pizza that contains 8 slices may have a serving size of only a single slice. All the nutrition facts shown would reflect the nutritional content of a single slice of pizza (or one serving size), not the entire pizza.
This definition may contain information from Wikipedia.