A serving size is a standard amount of a food set by the U.S. Government (FDA) or the manufacturer has chosen in order to describe the nutritional value of that food on the food label. Serving sizes are constant and are simply an FDA-recommended quantity to consume at one sitting or the manufacturer’s quantity used to display nutrition information. The FDA regulates what a serving size for a particular food is.
A portion is the amount of that food that you choose to eat at one sitting. Portions may vary from person to person or even from time to time for the same person! Ask yourself how many standard servings go into the portion you regularly eat? These 2 things may or may not be the same depending on what food you are consuming.

Attitional Resources:
http://edis.ifas.ufl.edu/fy823
http://www.eatright.org/public/content.aspx?id=4294967941
http://www.aicr.org/site/News2?page=NewsArticle&id=18113&news_iv_ctrl=2303