Unless you already know the size of your fingers, any jeweler would be able to measure it for you.

If you want to own a cheap effective tool to do so every time you need, here is a link of a small object you can buy on Amazon:

 

https://www.amazon.com/Measuring-Reusable-Finger-Measure-Jewelry/dp/B08JLW2C8J/ref=sr_1_5?crid=32B3OJDGQTLUP&keywords=ring+sizer&qid=1646604322&sprefix=ring+sizer%2Caps%2C226&sr=8-5