This can be tricky a tricky task, especially if you're trying to keep that element of surprise in tact! Most of my rings can technically be resized if necessary, but it certainly saves time and money to get it right the first time. Additionally, rings can only be resized to a certain extent. Depending upon the exact design you choose, that may be within about a 1 to 1-1/2 sizes in either direction. Feel free to shoot me an email if you're wondering the specific resizing range for a particular design. (Copy/paste the link to that ring, and be sure to include the finger size you're thinking of ordering.)
If you're completely unsure of her size and you're guessing, it's better to err on the side of a bit too large rather than too small. Sizing my rings down is both easier, and better for their long term durability. Sizing down means cutting open the back and removing a piece of gold, which creates just one solder seam. Sizing up, however, means cutting open the back and inserting a piece of gold, which creates a solder seam on either side of the insert. It's more work, and more expensive to size my rings up. So when hunting around if you're torn between two sizes, I would recommend opting for the larger size.
I wish there was a magic database of all people's finger sizes, but unfortunately, it's just old-fashioned sleuthing that works best. The best way is to enlist friends or family for help. Her mother or a close friend may already know her finger size. If not, ask one of her friends to take her out shopping one day to try on rings. It doesn't have to necessarily be "engagement ring shopping." They could be out doing something entirely different around town, and happen to walk by a jewelry store. Have the friend make an excuse about how she has to stop in to get her finger sized, since her mom is passing down a ring to her and needs to know for resizing purposes. While there, they can try on rings for fun and she can urge your partner to find out her size as well. Just be sure she's trying the sizing bands on her left hand ring finger (the wedding finger here in the US). Gathering sizes for fingers other than her wedding finger wouldn't help you much!
Those are the plans that I've found to work best, but certainly none are foolproof. And even if you do find out her size from a friend, just know that the ring may need resizing anyhow. Sizing is so specific, with factors such as the width and interior shape of a ring all playing a role in proper fit. For example, your girlfriend may find out her size for a wider band while out shopping, but not necessarily for my exact style of 2mm wide, comfort-fit Twig Band you plan to purchase. Saying to a jewelry store employee that your partner must be sized using a 2mm wide, comfort-fit sizing band could come off as a bit obvious or unnatural in that situation! But at least her friend's detective work will provide a good jumping-off point from which to start.
Another plan some clients like to use is to grab a ring of hers, and bring it into a jewelry store to find out what size it is. But my gripe with that method is that she's likely not wearing that ring on her wedding finger (since she's not engaged yet!). And all of our fingers are different sizes. Also, that ring could be much wider, or a different overall shape, all of which affects how a ring fits. However, this method could also give you a ballpark, starting point if all else fails. And then opt to have it resized after proposing if necessary. Resizing can be done locally at a trusted jewelry store or repair shop in your area, or you could opt to ship it back to me for resizing (for a fee). And during your detective work, I'm always here if you'd like an opinion regarding your findings before moving forward with an order.
What if you're not going for the surprise proposal, and you have the ability to find out your finger sizes together? ⇒ Lucky you! Take a peek at my FAQ's, where I go into more detail regarding sizing tips and considerations to make when stopping in to a local store to be sized.