I'm a newbie, and I've never had sex before, but as far as I'm concerned, a relatively average penis is good for the first time.
I would personally cringe from anything over 7 inches right now, but that's me, and I'm a very small person.
For those of you out there who are self conscious, please, don't be. Just take it slow, experiment, and learn how to use whatever it is you have been endowed with.
