It totally depends on what you want out of your marine tank. If you want fish only - then definitely the bigger the better because they all like space to swim around in.
If you just want one or two little fish and some live rock then you could keep it fairly small but minumum of 20 gallons I reckon.
If you want corals and mushrooms etc you could make a really cool little nano reef if you have the right lighting for them. Have a look on google images for nano reef to get an idea.
I have a 40 gallon with a few pieces of live rock, a clown fish, a damsel and a valentini puffer. I'm not gonna fill it with fish cos I like keping the stock low to give the fish a better quality of life but I wouldn't put much more in mine anyway. I will get aother clown to keep this one company and maybe a little shoal of 4
green chromis but that's it..
So what are your plans?