Well given that the average length of a full grown male's member is around 5. Now if we want to know if we've got a fat one we need to consider girth. The average girth is something like 4. You don't have to be above average in both departments to be considered "big.
10 Facts About Penis Size That Will Blow Your Mind | YourTango
How much do we really know about the penis? As much as we all take pride in this sometimes little package, many women and men alike don't really know all that much about it. And while ignorance can be bliss, knowledge — especially about that oh-so-favored of body parts — is most definitely power. While this may not seem entirely fair, it turns out that size can be a predictor of some matters related to sex, including a woman's ability to have orgasms during penetration. So maybe the idea of being a "size queen" might have some logic to it. BPEL and Bone Pressed Flaccid Length BPFL — is to place a ruler or measuring tape on the top side of the penis, pressing the base of the ruler firmly toward the pubic bone and extending it from there down to the tip of the head.
My last boyfriend had a long but super skinny one, which was weird. They all look pretty much the same and I think the difference is if a guy knows how to fuck. It felt like someone trying to fit their entire body inside me.