Most women do not wear their true or actual bra size (7 out of 10). But instead of it being because they just don't want to know, no one has ever told them how to figure it out. When we go through our right of passage, our mothers don't take out the tape measure and measure our rib cages add the 5, then take measurements from the tips/peaks of our breasts to determine the difference in measurement to determine the cup size. They visually "guess" what our size is, and then we're given our first training bra, we then spend the rest of our lives guessing, until we find someone that actually measures us for us.
You can do this at home and have someone you trust help you do this. If you don't have this as an option, there are specialty stores that can and will fit you. If you are in a country that sells and makes bras, then you should have access to a person that can size you for a bra.