Little BTAs always grow into larger BTAs. My 4" GBTA is now a 9" GBTA and growing. Personally, I wouldn't have a BTA in a tank smaller than 20g, and even then be prepared to upgrade tanks eventually, or trade the large BTA for another small one.
Here is RC's Anemone FAQ to get you started....
http://www.carlosreef.com/download/fclick.php?1