Using a pretty inaccurate method here to guess but, my thumb from the Proximal Interphalangeal Joint to the Metacarpophalangeal Joint is about 2 inches. Assuming this guy has a similar hand size to mine, which might not be true at all then his distances should be the same and then measuring the distance between those two joints on this picture we get about 1 inch, which would mean the picture shown here is about half life sized.
Then measuring the tip of his cock to just below his thumb, which is covering the base of his cock, I get 4.5 inches, so he should be around 9 inches. Now this second part might not be very accurate either since we can't see just where the base of his cock is either. Finally we don't know the angle of the picture to the phone, or the phone's picture to reality, and those angles might throw it off further but...
9 inches is as close as I can guess, 8-10 might be a better guess.
Edit:To speak to your question though, no this isn't average at all, it's way above average, average is around 5.1-5.5 inches.