There are smaller tanks than that, they are called nano tanks....some as small as 1 gallon. They can be really impressive. The thing you need to be aware of is the smaller the tank the more difficult it is to maintain water quality. Read about corals and chemical warfare between them to make sure you get corals that will "get along" with each other and do regular water changes of 10% and you will do fine. Good luck.
BTW a 55 gallon would be easier to maintain.