saltwater aquariums can be as small as 1 1/2 gal so it is up to you how much you want? Myself I started with a 55 gal and soon found I loved this hobby so much I went to a 200 gal. But I know folks that love it as much as I do and only have a nano tank. I can give you a basic set instruction if you want?
what ever size tank you decide you want. rule of thumb live rock 1 pound per gal. a protien skimmer that is rated for the gal size you want. just make sure it cycles good and there are no amonias nirtites or nitrates, from there it is just a matter of water changes every 2 weeks.