They average 3" in a home aquarium, but can double that in the wild, so occasionally you will see one get larger then 3" in a tank.
As stated, they are best kept in an exodon only tank and in groups of 10 or more. However, you won't need such a big tank. General rule of thumb with these fish is 1"/gallon however you can do quite a bit more with that. These guys do well in crowded setups.
Btw. One of my shoals is in a tank smaller then 40B.