If you plan to surprise her with the proposal, you want to make sure that the ring is the right size. A ring that won't slip on to her finger can really put a damper on the moment!
Since she is going to wear the ring on the ring finger of her left hand, you'll need to do your best to figure out what size that finger is. Remember that ring sizes vary for each finger, so only a direct measurement will be exact. But if you get it close, you can always have the ring sized up or down slightly to fit perfectly.

