It will take several years for a boa to become its adult size. Mine is somewhere around 4 feet long and about a year and a half old. A lot of the growth rate depends on genetics/feeding. A BCI is the best bet and you will most likely know if you see a BCC because its likely to be more expensive. BCC are less forgiving in their husbandry and if I understand correctly tend to maybe get a little larger. If you are worried about size go with a BCI male. As for the differences between BCI and BCC the saddles on the BCI look more like )( whereas on a BCC they typically look more like {}. I'm sure someone here with more experience can explain a little more.