*sigh* There is so much information about this on the internet, it's almost not worth replying to this question.
Haha no offense. Here's a helpful wiki article complete with creepy pictures of penises:
http://en.wikipedia.org/wiki/Human_penis_size
But I'll sum up most of what I've learned about the subject: MOST studies find that about 5-6 inches length is average.
But really, when people say size doesn't matter, they're mostly referring to LENGTH. A woman's vagina only has sensation about 3 inches in...so any weewee about that long will work for a woman. Actually, several women have told me that if a penis is too long, it will hurt them because the weewee slams into their cervix and causes pain. What's probably more important than LENGTH of a penis is GIRTH, or the width AROUND of the penis. The average girth also happens to be about 5 inches or so. Girth is measured by taking a tape measure, wrapping it around the penis, and seeing what number ya get.
I think every guy wonders about this at least once in their life...so there's nothing wrong with wondering or asking about it.
I'll leave you with one more tidbit: MOST, MOST, MOST women don't care what size their guy is as long as he can please her. Only shallow women have a preference for "big black dongs" or whatever. (which is a myth, by the way). I won't go into specifics, but I've seen plenty of black dudes naked and none of them have been monsters or clearly above average. So don't be a gorramned retard and repeat that myth to me because I've heard it so many times my head's gonna explode. XD haha
Well, there ya go. All you ever needed to know about dongs, straight from the horse's....
....best not to finish that phrase in this context.