The larger the tank is the easier it is to keep it healthy. However, it will cost a LOT more. You must be more precise with small tanks. I would personally start at least with a 40 gallon. What is needed, Live rock (approximately 1lb/gallon). A good protein skimmer, heater, hydrometer, and salt. Filter is not really needed, but a powerhead will help with movement. If you do want to use a filter, I would only use live rock rubble. Are you planning on having a fish only tank, or reef tank? Intense lighting is pretty standard with both types, although fish only can have just a T12 bulb on top. Key thing with starting a saltwater tank is to be VERY patient.