It all depends on what you are going to do, if you are talking only 1 or 2 taps, a 15 gallon batch will be fine, however history has shown that once it takes off, it will drastically need to be upgraded to bigger batches.
Also depends on how much time you have, if you are doing this on the side, then yes you will need a bigger set-up. IF you are doing it full time, a smaller one will work, but you will be using it more.
Dogfish Head brewed on a 10 gallon system for the 1st year he was in business at his brew pup, so it is possible.
Josh