And what she loves may not be so great on her finger - I have family rings I love, and they make my fingers look short and stubby and fat. They don't get worn much, but I do love them, just not on me. And since normally the ONLY ring we wear on the third finger of our left hand is the engagement or wedding ring, you can't get size from any other rings she wears. The fingers on the dominant hand are usually a bit bigger than those on the other hand, they have more muscles.
And if I had thought it through, I would not have decided on the engagement ring we got - I ended up never wearing it, because of gloves in the lab, and it was really too flashy (not a big diamond but flashy) for a grad student. When I started teaching, same thing - couldn't wear it at work. So it is worth thinking about - would she wear it all the time, or only for special occasions, once the wedding ring is on that finger.