A bra should lift and round a woman's shape without causing discomfort. The cups should not have denting, and the underwires should hold the breasts without poking in or sticking out. Cup sizes vary between manufacturers, and the band size gives a more accurate measurement.
The bra band should be snug and level, and it should be lower than the underwire. The bra should be chosen based on the outermost hooks due to the bra eventually stretching. The center panel of the bra should rest flat against the chest, and the straps should be comfortable and stay in place without digging into the shoulders.