A bust measurement of between 33 and 36.5 inches corresponds to a small, and a bust size of 50.5 to 52.5 inches corresponds to the largest size, which is a 3X. How does a 36 B cup bra compare to other sizes? While the B cup size is average, the bust measurement of 36 inches is slightly larger than average. Bra sizes are unlike most other garment measurements, in that they require measurements for the circumference of the body at two separate points.