Ring size is the one measurement you must obtain from a person you are actively deceiving, which makes it the most entertaining logistics problem of the entire project. Happily, the stakes are lower than they feel.
The clean methods
The borrowed ring remains the gold standard: a ring she already wears on the correct finger, taken to any jeweler for a two-minute sizing, or traced inside and out on paper with the diameter measured after. The accomplice is nearly as good; a sister or best friend can surface the number inside one shopping trip, provided the accomplice is chosen for discretion rather than enthusiasm. The comparison trick closes the list: her ring slid onto your own finger, the stopping point marked with a pen, measured later at leisure.
Choose the accomplice the way you would choose a safecracker: for nerves, not excitement.
Flying blind
If no method is available, the population is on your side. The most common sizes cluster in a narrow band, and the correct blind error is a half size large. A loose ring attends the proposal and gets resized the following week; a tight ring stops the show at the knuckle.
The net
Most rings resize by a size in either direction without drama, and reputable sellers include or discount the first adjustment. The exceptions are bands set with stones all the way around and tension designs, which resist alteration; measure twice before ordering those. For everything else, close is good enough, and the proposal should never wait on a decimal.
The short of it: trace a borrowed ring, or recruit a discreet accomplice, or guess a half size large, and let the free resize absorb the error.