I have always said that there isn't some kind of 'skeleton key' penis that any woman is going to love. Keep in mind the fundamental Penis Enlargement tenent that the ideal size is the one that makes you feel the most confident and happy. Fuck those hoes . . . just kidding.
8X6 is the conventional wisdom, but remember that's the true size of some of the most hung adult entertainment stars around (i.e. the top 0.01% of all men - aka freaks of nature - like a chick with natural DD tits, a flat stomach, hot face, a round toned ass, and nice looking pussy - how many of those do you see walking around?). The average woman will have never seen anything close to that size (despite what they may say), and you really don't need it to sexually satisfy them. It's the best size if that's what you need to feel confident and happy with yourself. Otherwise, it's like saying you hhave to be 7' tall to be able to play basketball. Sure it helps, but how many guys are that tall, and how many skills do they normally have at that height?
As another poster said, and I can confirm with personal experience, anything around 7" is going to be running too deep on a good number of girls, and pleasingly deep for the rest. So far as girth, there are conflicting estimates about what's best, but at least a nice filling feeling is what you're looking for. On Penis Enlargement sites you're going to hear a lot of guys say that you need to stretch them to insane levels to be satisfactory, but that's just the natural locker room product you're going to get in an environment like this.
In reality, vagina's vary in size and width as much as dicks, but generally stay within a median range, just like dicks. What really factors in is how much she digs you and how turned on she is by you - just like if you're into a woman, you won't mind if she doesn't have the snuggest, most tight pussy you could imagine - she won't care that you're not jamming a maglite into her birth canal. So, girth, like everything else, is relative. Aim for what makes you happy. Some girls can take a fist in their snatch without orgasm but cum hard from some concentrated g-spot or clit attention - girth is one component in a huge field of variables. Don't oversell yourself on it.
At an 8X6 level size you have almost a 100% chance of being the biggest dick any girl you fuck will have ever seen. If you need that to feel okay with things, then go for it, all out. If you're 7X5, any woman is going to consider you to be pretty well hung unless she has a cow pussy or some freakish size fetish. If that's what you want - go for it. Anything below 7X5 but above maybe 5X4.25 is well within the average range and she isn't going to look down on it. Like I said, if you make her hot, she's going to want it regardless. Certainly the first thing going through her mind won't be, "ahh, sub 8X6? Damn, what a loser!" They don't think in terms even close to that - in most cases they could really actually even give a shit.
If you have a size you're happy about you're going to be more confident and hence more sexy to women period - make that your priority.
And that PenisSize Debate site is total bullshit - sponsored by enlargement and adult entertainment sites. Calling the science the author employs amateurish is a compliment. His 'ideal' size range is enough to rip most vaginas to shreds during normal sex. It caters to masochists and morons exclusively. Just read the commentary section if you want to get an idea of the caliber of person that takes that bullshit seriously. It's a buffet of retards, sub-literates, and anonymous gay cruisers . . .
working on the girth now. starting to fatten up 
:s