Size is related to age and childbearing history as these can affect pelvic floor tone. Size is not related to menstrual flow.
Size 
Recommended for those who are 30 and over, and all who have given birth vaginally regardless of age.
Size 
Recommended for those under the age of 30 who have not given birth vaginally.
The difference between the ‘A’ and the ‘B’ is only small (3mm), but it is crucial!