The average womens vagina is 3-4 inches deep. However during arousal, it can expand a little more. So any penis length over 6 inches should be adequate to fully be inserted to a vagina and hit the Gspot. Any more than six inches doesn't really do much for sexual pleasure since most of the vagina is already filled. Granted a 9 inch cock is nice to look at though.
What separates men from the boys is GIRTH or thickness. The average male penis girth is 4.6 inches around when fully erect. A thicker than average penis will fill up and expand the vaginal walls to cause increased sexual pleasure. Ask any women who has tried both and see what she says matters more - length or thickness? A recent study showed that 75% would rather have a 6-7 inch length cock with at least 6 inches of girth vs. a 8+ inch cock with only 4.6 inches of girth....
Of course knowing how to use your cock when fucking is important also!
Ladies - anyone want to chime in on length vs. thickness?