"Big" and "small" are relative terms... and might I add, relatively irrelevant.
So, one must ask, "big" or "small" according to who, and compared to what? I knew a guy with a penis so small it was barely visible... it looked like nothing more than a small protrusion of foreskin. Admittedly, I felt sorry for him and his future wife. He was 17 at the time (I was at college).
I know another man with a penis so large it quite literally comes almost up to his collar-bone (upper chest) when erect. It is surprisingly enormous -- has to be seen to be believed. Whilst I don't feel the same sense of sorrow for him, I can certainly imagine that having sex is pretty much out of the question for him.
If it's sex your concerned about -- and I imagine it is, because any size penis can urinate, and any size penis can be self-stimulated... other than that, what else would you use it for? -- well, both of the above mentioned gentlemen have an impossible time in that regard. With your 6in friend, I am sure you'll be just fine... keeping in mind that there is just as there is a lot of variation in the relative size of the male penis, there is also a lot of variation in the relative size and tightness of the vagina.
Some woman might find your 6in one-eyed-wonder too small for optimum pleasure. Other woman The vast majority, however, are going to find it absolutely fine... at least from a purely physiological point of view. As "Monkeynunky" pointed out in a comment above, it tends to be the girth not the length that's (relatively) more important. My penis happens to be on the longish side (about 7.5-8 in) and some women find it goes too deep which can be slightly painful. But that's not such an issue, as I just have to be mindful and not go all the way in. Woman I know have mentioned that some men have a very thin (small girth or diameter) penis and they can hardly feel it. These women, mind you, also tended to have a larger vagina so they are more likely to notice that.
If your sexual preference is men (I don't wish to make any assumptions either way), I suspect smaller is tending toward better. I understand many homosexual men don't actually engage in anal sex (or very rarely), but for those that do, the anus is relatively tight and small compared to the vagina. So if that's your sexual orientation, I doubt your 6in friend will give you any issues there.
So... perhaps by now you're starting to get the gist of this. It's all relative and highly subjective... based on many factors. The most important factor, in my opinion, is this:
How you feel about yourself and your penis... along with your (and your sexual partner's) willingness to be with each other in an open and honest way is what matters most. That can be very challenging and awkward at 16... even at 40... but more so as a teen. If you meet a woman who is not an "ideal" match for your penis size I would think it's only a matter of open communication and you'll find plenty of ways to make it all work out fine, and feel fantastic for you both. As to what an "ideal" match is... well... highly subjective, that is. So again... it really doesn't matter.
Go forth my pubescent male friend... go forth with your 6in friend... use him wisely... kindly... passionately... and most importantly... LOVINGLY.
All will be well.