Wider bands fit more snugly than thin bands. If you are planning on a thick, chunky setting, you may need to go up a quarter or half size. Choosing a Resizable Setting
If you want to keep the proposal a secret, you’ll need to be resourceful.
Ultimately, while you want to get as close as possible, don't let the fear of a perfect fit ruin the moment. Most jewelers offer one free resizing after the proposal, ensuring that once the "Yes" is secured, the fit can be perfected.
Fingers tend to swell throughout the day. It is best to measure in the evening when the hands are warmest.