when I purchased my wife's ring, I never heard a comment about size, it was all about "so and so said they saw it sparkle from across the room" etc. etc. It's all about clarity and color, that's what sets the nice diamonds apart from the rest. I think I got her a 3/4 ct with 1/2 on either side or something like that. Nothing huge, but tasteful. We spent about 4k-6k on it, I can't remember exactly what.
It all depends on how much you're willing to spend and also what's important to her. Keep in mind you are using this ring to ask her to spend her life with you, it says a lot. No pressure :thumbsup: