Why any guy would still marry the kind of selfish broad disappointed by the small size of the engagement ring makes me shake my head in disbelief.
When she considers the proposal went wrong because of the size of the ring, the fact that he didn't get down on one knee, or failed to come up with a flash mob, this should set off an avalanche of warning bells screaming at you to NOT MARRY THIS PERSON!!! When you marry someone, you need to be on the same page.
On the other hand, sometimes life happens and proposals go wrong. Maybe a bird craps on you right as you try popping the question, you get swamped by a wave while on the rocky outcropping by the sea, or nerves make you barf right in front of her before you ask the all important question.
If any of those things happen, you know you found the right person if they laugh it off and declare their love for you regardless.