According to the search results, the smallest bra size is generally considered to be 28AA, although some specialist retailers produce AAA cups as well. Bra sizes are determined by two measurements: the band, which is the measurement around the ribcage where the bra underband sits, and the cup, which determines the volume of the bra. The cup sizes are arranged alphabetically, with AA being the smallest cup size. Some brands may not stock sizes below A, but there are expert bra-makers that stock AA and even AAA cups in band sizes 32 up to 36.