There's a few ways to determine your ring size. You can have a jeweler check the size of some of your favorite rings for you and write down what sizes they are. There are printable ring sizers online that you cut and place around your finger to measure it but you have to make sure your printer isn't making any size adjustments to the page. If none of those work for you, you can also purchase inexpensive ring sizers like the one we sell here: https://www.azureaster.com/ring-sizer.html
Jewelers sample rings are size 7 for women and size 10 for men. So if you get really stuck those are considered to be the most common sizes for average framed folks.