Going along with PonieGirl's answer, sites that sell fish generally list the size the fish is at that price, not how big he will get, and the minimum tank size is what he needs at that size, not his future requirements.
ask questions before you buy something to see if it will work. Instead of after you bought it, wasted your money too find out that it won't work or asking how to try to make it work. the people on these boards dont make one thin dime off of your purchases so they will tend to steer you in the right direction instead of someone tryin to sell you a $200-300 piece of equipment that you may not even need.