Average Erect Girth is measured at the mid-section of the penis. To determine your average erect girth size you will need a tailors tape or a piece of yarn or string. Stimulate yourself to achieve a full erection, once you are as hard as you can get, kegel as much blood into your penis and take your measuring device and lightly wrap it around the circumference of your penis. Be sure that it is touching on all sides but do not pull tight. Mark off your girth at the closest quarter inch.
Keep in mind that the average erect girth is about 5" in circumference. Increasing your girth size to a larger average penis size can be accomplished through the exercises found on Matters of Size Penis Enlargement site.