You want your ring to feel comfortable and fit over your knuckle without too much difficulty. And it should fit your finger well enough so it wont fall off.
You can easily determine your own ring size with a measuring tape or a cord. To measure simply place the cord (or the measuring tape) around the finger where you want to put the ring. The ends of the cord should be marked. Use a ruler to measure the cord and determine your ring size in mm.
If you are still unsure of your size you can also measure the inner diameter of one of your own rings and refer to the below measurements.
In case the ring of your choice has a wide band around 7mm you should choose two sizes up.
If your size is not available, please don't hesitate to get in touch at susandtainstore@gmail.com and we will see if we can get a custom ring made for you. 

US

EU

UK

Diameter

3

44

F

14mm

4

47

H

15mm

5

49

J

15.6mm

6

52

L

16.5mm

6.25

52.5

M

16.7mm

7

54

N

17.2mm

7.25

55

O

17.5mm

8

57

P

18.1mm

9

60

R

19.1mm

10

62

T

19.7mm

11

65

V

20.6mm

12

67

X

21.3mm

13

70

Z

22.3mm