No one person can prove that size does or does not matter because it is a matter of preference. And I don't know who has been lying to you, but you can feel way more than the first two inches! And maybe she does like his pelvic bone on her clitoris or maybe she just wants it to go deeper! I am sure that she realizes exactly what it is that she wants. She doesn't need someone else telling her. Size does not matter to a lot of women but it does matter to a lot women.
Every women (and man) likes different things sexually and no one can say what someone else wants or likes. He will find someone who absolutely loves his five inches! But some women just want more!
Reading books and such to improve your sex life is fine and can be helpful, as long as you keep in mind that everyone likes something different. You can read as many books as you like, but if you start living by them, you may end up sucking in bed. Confidence is one thing (and is necessary) but believing that you are "the man" and that you can please any woman will probably fail you. Simply because to one woman you may be "the man" and do exactly what she likes when she likes it, but to another woman those same things may turn her off!
Honestly, do not worry about your size because five inches is going to make whomever you end up with for the long haul very happy. As you have read above size really does not matter to a lot of women. It is probably about a 50/50 ratio. So there are plenty of women out there who will be quite pleased with you. And the other 50% will either learn to love it(if they are worth it, and care about you) or if it is that big of a deal they will find a way around it.
I know that not everyone agrees with me and that is absolutely fine, but no one can say for sure that size does or does not matter, or "prove" it. It is a matter of preference.