Are you asking a serious question or just getting off with a fantasy?
Let's assume the former and be realistic:
You said: "but I just get the feeling that a huge cock would drive her crazy". Is this her fantasy or yours? Most women actually don't care about size unless you are abnormally small or abnormally big. At 6.5 inches, unless you don't know what you're doing, you're probably satisfying her as much as a larger cock will (and larger may only provide discomfort).
If you think that she wants larger, ask her. If she wishes to try someone bigger, then by all means proceed. If this is all your fantasy and she agrees to acquiesce to your fantasy, then by all means proceed. But what's the point if neither you nor she wants "bigger"?
By her age, I assume you're also young. Young and unmarried, you're playing with fire. It's a dangerous game to proceed if you and she haven't a firm foundation (and 4 years of dating doesn't count as firm foundation--sorry, not trying to be mean, just honest).
If you are prepared to lose her if this all blows up in your face, then by all means proceed. But be aware, once the deed is done, it is impossible to undo the past. You can only go forward.