Finding the right ring size is a task of its own, and it can be stressful to wonder if a wrong size will ruin the proposal - but we've got your back.
The best way is always for her to go to a jeweler to get her ring measured, but if you are to surprise her with the ring, that might be too obvious of a choice. So, here's an alternative: find a ring of her own, which you know she wears in her left or right ring finger, and place it over a ring size chart to compare the inner diameter.
Click here or in the following image
to download the chart as a PDF, and print it out full scale in a letter-size sheet. Compare her ring to each size until you find the one that fits completely inside the ring. Good luck!