The differences with BCC/BCI involve scale counts, saddle counts, and locality. Size has less to do with BCC/BCI and more to do with locality. As far as purchasing, as a general rule your regular red tails are usually BCIs, and your locality boas are BCCs. Females do get significantly larger than males. Like Andrew said, they are fairly slow growing snakes and take several years to mature. A 3x1.5 is fine for a 3' snake.