How do you know her finger size without asking??

Does she have any other rings that she wears? You might borrow one of those without her knowing to get the sizing of that one, and base the engagement ring size off of that. If it doesn't end up fitting, you could always get it resized later.

If she has any rings that she wears regularly you can sneak one of those to a jeweler where they can tell you the size (this is what my fiancé did!). You can also ask friends/family if they might know what size she wears. Last but not least in the slightest- you really just want to get close. Most rings can be resized, although it is often easier to resize down rather than up.

I agree with PP, getting close is really what matters. You can always take a photo of her hand while she sleeps to help the jeweler get a rough estimate. Another thing I've hear of people doing is getting some yarn/string and while she sleeps wrap it around her finger and then mark with a marker on the string where the ends meet. The jeweler can measure that to get close to her size.

Ask her friend to help. Ask her friend take her to a ring store "for fun", carefully not letting her know. And she will be professionally sized and you just ask her friends for the answer!

I secretly bought an engagement ring for my wife and gave her a surprise proposal. I tried several times, using a string to measure her finger when she's asleep. But I was so nervous and never made it!! Then I bought my mom a ring, and I asked if she could help try the size, and I got her finger size. The point is that you have to act supernaturally (you're just choosing a ring for your mom!!) so she won't get so suspicious about that. Hope your succeed

If she had another ring, borrow it before she's aware of it. I did this. And I used that ring to compare with the ring size chart on the online store. Not so perfectly fit, but my proposal really surprised her that day. We made a slight change of size in a craftsman shop.

The others have given you loads of great advice about how to get the size, just to add that one ring type that either cannot be resized or cannot be done so with any ease is a full eternity band with diamonds that go around the full ring, like this diagram shows:

1/2 or 3/4 eternity bands are much easier to alter the size of.

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Agree if it does not end up perfectly fitting, you can get it resized later. or you can just propose with a simple gold ring, and then choose the diamond ring together in case she has already decided what she wants.

I actually bought a ring sizer from Amazon, the type the Jewlers use and took one of my fiancés daily wearrings and measured it myself.

I made this surprise proposal to her last year! I'm telling you, her best friend is the best choice. She definitely will help you get a very accurate number. Her best friend helped me take her out shopping, and they went to a jewelry store for birthstone rings. They tried multiple styles, and her size was secretly recorded. And then the most challenging thing was to choose a special engagement ring. I did consider Tiffany, but their rings were beyond my budget. Then I heard a brand - Darryring, which you can only customize an engagement ring for one & only in a lifetime. I chose my heart collection, the price was fair enough. I signed the true love certification and they sent the paper to me. The proposal was perfect! I recalled all the beautiful memories and asked her that question. She signed her name on that certification paper and said yes! Thanks to her best friend!