There is a great variation in penis size among mature men, and even more so among adolescents. At age 17, some have attained almost full adult growth whereas others are in an earlier stage of adolescence. Also, if you read about penis sizes, there is a lot of conflicting information because of the different ways that researchers make measurements. The following is information from an apparently well-conducted study using a large sample of thousands of young adults; use the following link to see the entire report. (Please note that it is a survey, and not a clinical study.)
A typical 17-year-old is going to be only slightly smaller than these numbers, especially for erect penis size; but remember that "typical" and "average" are functionally meaningless statistical values; what you've got is what you've got, and making a contest out of body features is immature behavior.
Length of Flaccid (soft) Penis: Typical Range -- 5.9 cm to 11.3 cm -- 2.3 inches to 4.5 inches Shortest -- 2.5 cm -- 1.0 inch Average -- 8.6 cm -- 3.4 inches Longest -- 15.7 cm -- 6.2 inches
Length of Erect (hard) Penis: Typical Range -- 13.0 cm to 19.2 cm -- 5.2 inches to 7.6 inches Shortest -- 6.8 cm -- 2.7 inches Average -- 16.1 cm -- 6.4 inches Longest -- 26.0 cm -- 10.2 inches
An erect penis thickness is typically 3.2 cm to 4.8 cm -- 1.3 inches to 1.9 inches.
("Typical" roughly means the central 50% of the survey group, a fairly reliable quantity.)
One interesting result from penis size research is that there is frequently no proportionality between flaccid size and erect size. A person with a shorter flaccid penis will often have the same or greater erect length compared to a person with a longer flaccid penis.
Finally, a penis will appear to be shorter than it's actual length if there is any fat in the groin area at the base of the penis' shaft.