loooads of different bichirs that are commonly available in the trade...
if you've brought one then post a picture and someone will be able to identify. if you havent then maybe back to the LFS and write down the name. a quick google will find the information that you need though. and there are also profiles here : http/www.fishforums.net/index.php?/forum/44-oddballs/
tank size is obviously dependent on species, and maximum size.