The general rule of thumb is:
Size ‘A’ is for under 30’s and who haven’t given birth.
Size ‘B’ is for over 30’s or who have given birth.
However, base it around your needs as you know your body best – if you’re a woman, age 32, never given birth and have fairly ‘normal’ period, go for the size ‘A’. If you’re 22, have really heavy periods, then try size ‘B’.
The most important thing is to understand the length of your vagina and this can be met by snipping the stem of the cup to suit your cervical height.