3ft x 2ft x 1ft would be about 44 gallons.
The suggestion made above is quite correct and is very much on the safe side I think. I usually go by about 20 gallons of water (75 litres) for a fully grown fish. I think 44 gallon aquarium should be fine for your use but if money and space is not an issue I would suggest 55 or 63 gallon tanks. It just saves you from having to upgrade later on. And bigger the aquarium, more impressive it looks. I am currently looking into getting a 70+ tank but being a college student, I find it difficult to get the funds for it. Bummer. Anyways, enjoy.

bb