Bigger is better, but if you do your research, a 10g can be a starting point.
Do you have a local friend in the hobby? A mentor will go a long ways...get yourself a book or two on SW tanks, and also check out wet web media online before you do anything...once you read up, you can ask specific questions here, and get some good answers!
Get your sand and LR, and start doing a fish less cycle...that is once you have all the hardware together for this tank.