well said, reef-addict, but to answer the question it can be done, nano-reef.com(think that is the address), has them as small as 3/4 gallon, the sight is awesome for mini reefs
if i were gonna do a nano reef, it would not be my first tank, and i WOULD have as large a sump as i possibly could, to help keep and maintain stability in the water(the sump can be larger than the aquarium too, if you want)
HTH and good luck