Which size cup would work best for you, depends on how deep your cervix is. You can check this in the shower, when your hands are clean. Your cervix feels like a small, slightly hard bulge that springs back to the touch. If you can already feel your cervix when you’re one phalanx deep, your cervix is low, around your second phalanis average. If your finger comes even deeper than that, your cervix will be a bit higher. It is best to check this every day of your period, so that you can determine the lowest point. The higher your cervix is, the larger size cup would be best. Size 1 is 56 mm long and has a diameter of 42 mm. Size 2 is a bit longer, at 58 mm, and is 48 mm wide. 

The volume also plays a role. If you have a heavy period, it is best to opt for a cup with a little more volume, as you won’t have to empty it as often. The cup in size 1 has a capacity of 22 ml and in the size 2 fits 29 ml of blood.