Good sex is more about technique and ability than about pure dick size. 6.5" is fine, but can you use it? And how long can you last? Takes on avg. probably 15-20minutes of stimulation for a women to get off, whereas a guy can do it in 2-5min if he's a NEWBIE. Learn dick control, how to give really good oral (duration matters there too), and how to be warm her up with alot of kissing, licking, etc... before hand. Otherwise, even if you had 9", its not to interesting for her if its over in 2.5 minutes.
DrG