They vary in size quite a lot. Most of us have one hanging lower (usually the left one) than the other and, strangely, the higher is usually larger than the lower. Well, that's what I read on the subject a few years back. I don't know what the average is but to get things rolling I just measured mine with a vernier gauge and the left (lower) one is approximately 2" (51mm) long and the right is 2 1/8" (53mm) which bears out that theory.
My first regular girlfriend (many many years ago:s) told me that she thought I had big balls. I never knew if that was a kind way of saying she wasn't impressed by my cock size (she didn't comment on that) or if I really did have big balls.