A bra should fit well. An ill-fitting bra is uncomfortable when you wear it.
When you are large breasted an ill fitting bra can cause pain between the shoulder blades, at the base of the neck and back pain. Research has shown that most women are wearing the wrong size of bra, mostly one that is too tight. They are often forced into a posture which is unnatural and unhealthy.
A well fitting bra also looks better. So measure up properly before you buy a bra and always try it on first. Take a tight fitting top with you to check if the bra enhances the look of your breasts and the seams and lace do not interfere with the profile.
Bra size consists of two parts: a number and a letter. The number has to do with the ribcage and the letter with the size of your breasts. Measure around your rib cage, directly underneath your breasts. Measure the breasts by holding the tape across the fullest part of your breasts. Don’t hold it too snug for that will compress your breasts and give a false measurement. The difference between the two measurements is your cup size.

Measuring up: Dutch sizes & English sizes
Three common problems with a wrong fit