The smallest bra size is AAA. An AAA bra is less than 1/2 inch larger than the chest at the fullest part of the breast.
Band sizes, which are represented by the numbers in a bra size, are the base in determining cup size. For example, a woman who needs a 28-inch band to fit around her ribs, and a cup size that measures 28.5 inches or less at the fullest point, would wear a 28AAA bra. An AA cup bra would be 1 inch or less bigger than the band. After that, cup sizes go up in 1-inch increments. For example an A cup with a 28-inch band would measure 29 inches at the fullest point, and a B cup would measure 29 inches.