Ok to clarify most men think about penis size more then most women do. Its like how most guys think about tits, big ones, medium ones, small ones as long as she has them they are perfect. My best word of advice would be for you to perfect your oral techniques, as most women do not cum by penetration alone. As for the size the smallest I've ever seen was 3 1/2 inches when fully erect so you are huge, you with your lengthy stiff 6 inches. And I've spoken with a friend of mine who dated a guy who was smaller then that and about as big around as you baby finger when hard. Now granted these are not average sizes, but you have nothing to worry about with 6 long hard inches on your side.
As for not lasting. It happens. If it happens on a regular basis you should spend some time trying to find out what the cause is, but if it's only happened the once don't over analyze it or you will find yourself with a sexual hang up from being too stressed out about going off to soon. Things like that tend to snowball on you. If it happens again take the time to really concentrate on your woman, make her cum either orally or manually then go for the second round, you are young and should have no problem getting it up again. Just don't leave your woman unsatisfied.