The Bigger the better with saltwater tanks. Nanos aren't the best idea for someone just starting off because of the fact that the smaller volume of water, the easier it is to mess it up. I suggest buying a few books and learn as much as you can before you get into it. Make sure you know about the cycling process. Dont be under the impression that you can just throw some saltwater in the tank and fish and you're set. Saltwater tanks take a couple of months to really set up. If you're going wit ha fish only aquarium, it is recommended you get live rock, but not necessary, the same with the lighting, NO (normal output) is fine for fish. I suggest buying a reef ready tank. This is a tank that comes pre drilled with an overflow for the addition of a sump/ refugium. I really wish i had made the purchase when i started my first tank.
Some of the best advice you will receive is to take it slow. If you are going to get a tank larger than 20 gallons, i would definitely get a skimmer. Also you cannot get as much fish in a saltwater aquarium as you can in a freshwater. An approximation of the amount of fish you can have is 1 fish for every 10 gallons or so. This obviously will vary depending on the size of fish. So if you get a 10 gallon, you can have 1 small fish, no more than 2. For sand you can use live sand, but that can be expensive so i suggest using aragonite. You can always add some live sand later on.
there is so much i have left out, so keep asking questions, there really is an endless amount of information you can learn about this hobby.