Serving size is really the average portion that one should take. For example, a pizza serving size is generally one slice. A. banana serving size and an apple serving size would mean a single banana or a single apple.
An oatmeal serving size might be a cup, while a cake serving size might be one eighth of the cake.
Packaged products also provide labeling information to indicate the serving size of a food. For example a 16 ounce bottle of Gatorade might have two servings which is a little bit misleading because most people drink a full bottle when they are thirsty and don't realize that they are drinking two servings rather than one.
Having larger portions often contributes to dietary problems which result in increased weight and possible obesity problems. Adhering to the actual serving size and accounting for the calories consumed helps to keep your weight in check.