In response to the comment about small tanks being difficult . . . . this is a true statment but as with eerything in life, there are limitations.
A 20 gallon tank IMO is a good tank size. It is going to be more challenging in some aspects and easier in others. For example, you have to pay much more attention to water quality, but you should do this regardless of the tank size. On the other hand, water changes are easier because they are a smaller volume of water. just as an example.
I do say to get the largest tank that you can afford, but a 20 is a great place to start IMO. It really "trains" you in many aspects of detailed reefkeeping and can lead to a more "aware" hobbiest. Once you get then hang of running a small tank then going larger is MUCH easier. However starting large and going smaller is a much more difficult transition.