The best way to know your finger size is to go to a jewelry store to have them take your measurement and give you the exact millimeters.
If not:
How to know the circumference of your finger (the ring size):
Step 1 : Wrap the finger with a piece of string or paper (Tighten a little).
Step 2 : Make a mark where the paper overlaps the one underneath.
Step 3 : With a ruler, measure the distance between the two lines.
If, for example, the circumference of your finger is 6.3 cm, the size of the ring is the same, but expressed in millimeters (63 mm).
In this case: 65 mm