cbwmn
Aquarium Advice Freak
I think for a beginner, 1 inch per gallon is a good rule to live by. This rule can be stretched if you add live plants to the tank, increase the frequency of water changes, upgrade to awesome filtration (canister instead of hob).
Personally, in my 10G tank I've got about 22 inches of fish (guppies, mollies, platys), but I've also got about 2 pounds of anacharis, java moss and christmas moss in the tank as well. Nitrates are ~4ppm, nitrites and ammonia are 0ppm. I also do about 30% water change about once a week.
If you put a 5 inch Discus in a 10 gallon tank, it would not live long.
You also have to consider body mass.
Charles