They should come off your finger with slight resistance at the knuckle, otherwise they might just slip off without you noticing. Your ring size will vary slightly throughout the day due to changes in temperature, blood pressure, and water retention among other factors, so it's best to pick up a cheap ring sizer like this one and wear one of the loops around for a day to see how your size fluctuates. The difference between sizes are fractions of millimeters in diameter (about 0.8mm difference between sizes, and ring sizes can get as specific as quarter sizes) so I think you might continue to run into sizing issues if they only offer rings in sizes of whole millimeters.
We walked into a Hezelberg Diamonds in the mall and asked to have our fingers sized. I then tried on different settings and diamonds shapes. That's how I learned I didn't like halos as much as I thought I did and that I loved a simple solitaire.
I also bought a ring sizer off Amazon https://www.amazon.com/dp/B08THGJJ4D?ref=ppx_pop_mob_ap_share
In the photo it looks snug but not TIGHT. You mentioned being able to turn it, does it turn easily? If it turns easily/ without struggle you might be a 7.25 or 7.5.
I bought off Amazon this sizer, https://www.amazon.com/dp/B08THGJJ4D?ref=ppx_pop_mob_ap_share
I had been sized at Hezelberg previously as a 4.5 and this sizer provided the same results.
Also there's standard fit rings and comfort fit rings. For comfort fit rings you'll want one size smaller than your standard size (so if you're a 7.5 then you'd want a 7.25 for comfort fit). https://www.camoeverafter.com/comfort-fit-vs-standard-fit-rings/
In that case!
All our fingers vary :/ like my ring finger is a 4.25 but my middle finger is a 5.5, pointer finger is a 4.75. You can buy one of those metal ring sizers on Amazon and measure it to that ring you currently have, I imagine it's bigger than her ring finger?
KIWEN Ring Sizer Set Metal Finger... https://www.amazon.com/dp/B08THGJJ4D?ref=ppx_pop_mob_ap_share
If you can determine what size that ring is, and it's bigger than her ring finger then estimate like half a size smaller or .75 smaller. Remember that you rather have a ring that's slightly too big than too small. And certain rings can't be resized due to their structure, like certain pave bands or tension settings.
You might want to have some of these on hand when you propose so if it's too big then she can immediately make it fit better that same day! Ring Size Adjuster for Loose... https://www.amazon.com/dp/B0757JLTY7?ref=ppx_pop_mob_ap_share