The size of a bra cup a girl should wear if the measurement of her bust is 4 inches more then the measurement of just below her bust. It's a matter of taste if this is too big, not big enough, totally gross or the most fun you've ever had.
Shouldn't cause back pain but sure creates a nice cleavage!
Calculate your cup size. Subtract the band size measurement from the bust measurement and determine your cup size as follows: (The cup size calculation may be less accurate for larger cup sizes.)