The serpent ring is made to be slightly adjustable, in order to fit multiple fingers, and to accomodate for any swelling or size cahnges due to the weather, arthritis, pregnancy, etc. Although the ring looks large enough to make a statement, the band still fits as comfortably as one of ARJ's more dainty stacking rings.
 
Band is about 1.5mm in size, and serpent is around 3/4"x3/8".
Sizes are determined as follows: 
XS- 3 to 4.5
S- 5 to 6.5
M- 7 to 8. 5
L- 9 to 10. 5
XL- 11 to 12. 5