When doing the "1 inch of fish per gallon" rule, don't use the fish's
current size. Find out how big the fish will be when fully grown and use
that size. It may look silly to have a big tank and two tiny fish, but fish
grow and in a couple years it won't look silly. Also, some fish have
special requirements (very messy, very active, or should be kept in a school
of 6+ fish, etc...) and may need bigger tanks. Just as stress can make a
human sick or even die, fish can be stressed by inappropriate tank setups
and get sick and die too. Research the fish before buying it.

http://fins.actwin.com/ is a good place to start.

E.Otter