Check out the smallest tunze, it's like 8 inches high and 2.5 inches wise.. It runs pretty well too, though you might go with the 2nd lowest size, unless you go with an external or such.
I would caution against any fish in the tank too, it's just another nutrient source. (and waste != food)
Maybe get a peppermint shrimp for any aptasia that might come along, hermits/snails for any algae. If you go the fish route, I know the sixline wrasse is popular for taking care of some pests.
And go with led's. You've got some reading to do, but it's worth it. (and only do sand if you can do larger weekly wc's or carbon dose/nopox)