I've seen the average size stated as between 5-7" for Americans. There are some exercises that one can do to increase the size of your member that I have heard word (but have no personal experience with) and these are called "jelqing" or "jelquing." See this link: http://www.altpenis.com/penis_news/jelqing.shtml
Also some guys who were circumcised go as far as regrowing their foreskins which has been reported to increase the girth and sometimes the length of the penis, and also increases sensation for the man (and his partner).