I would go with the full 15 gallons if that's your limit for now.
I have to ask how much to you know, or understand about saltwater setups?
you will need about 1-1 1/2 lbs of live rock per gallon, this is the best filtration, a protein skimmer for a tank this small is impractical, mostly the models that are the right size are terrible and have to be heavily modified to pull any skim mate darker than a light tea.
you will want to use a fine aragonite sand, an inch or so deep. (I'm assuming this is for a dorm room?) if not make the sand 3-4" deep.
you will need water movement, a powerhead or 2 will be great.
are you planning a fish only, a light reef, or a little slice of the reef?
I ask for the lighting is complex and it all depends on the aim of the tank, say you only want a little clown goby and a pair of percula clown fish (very doable in 15gls) with a few zoanthids and Xenia, a 2x40 watt satellite would work, but if you wanted an sps, lps dominated tank you would need T5, VHO, or Metal Halide lighting. like I said complex.
you will also need the equipment to maintain you system, like a refratometer, or hydrometer. tests for the nitrogen cycle (ammonia, ect) calcium, alkalinity and so on.
good luck, and if you have any questions I will try and help as best I can.