One of my dearest friends became engaged over the summer. Her now fiance and their best friend (also a sister) went shopping together. She got a 1/3 carat and it is the perfect size for her hands and looks stunning. He opted for a "Hearts on Fire" which is one of the supposed "perfect" cut diamonds. And honestly next to any other sisters rings, her's sparkles like no other.
I think it's the love in the relationship which makes a marrage work, not the size of the ring. And I think that actions speak for words that he tried to find her a "perfect" ring. I just think it's so sweet of him. FYI he also got her the matching "perfect" earings for Valentines Day!
I personally would like a 1 carat, but I would be happy with anything from someone I loved enough to spend the rest of my life with.