The most reliable way to find out your finger size is to go to any jewelry store, where you will be measured for free. We'll advise not to take measurements on cold hands, first let them come to a normal state at room temperature.
If you want to make a surprise for someon, you need to "steal" a ring from the that person and take it to jewelry store for measuring.
Or you can use home methods, but be careful, because even the small error in the measurement and the ring will fly off your finger. If you make a mistake with the size - resizing is possible with additional fee at our workshop in Moscow, Russia (delivery fees are on you).