It has to be big and she must be able to brag about it...And, she must also have a large diamond to flash around.
My wedding set has the smallest diamond imaginable. I just bought myself a replacement that I can wear for every day without snagging. Cost me $50 at Walmart.
And I'm in love. Fancy that.
(FYI, my daughter is hoping her future husband buys her set off Ebay. "You can get a good ring at a great deal. Anything else is a waste.")