You can use a combination of several factors to figure out what size cup is the best fit for you. According to Kecia Gaither, MD, director of Perinatal Services/Maternal Fetal Medicine at NYC Health + Hospitals/Lincoln in the Bronx, the size of the menstrual cup you need is based on physical features such as your age, cervical length, intensity of flow, reproductive history, and the strength of your pelvic floor musculature. However, she adds that you should also consider the firmness or flexibility of the cup (you may want a softer or firmer feel) as well as the cup capacity (i.e., whether you need an extra-large cup that holds more than the average amount).