Better questioned in the salt water forum....
Any hoo.... This depends greatly on what corals your going to keep in this tank.
I'm going to assume your are refering to a Nano cube or similar. Your going to be very limited on your options. Most people either buy P.C. lights made for these tanks or buy P.C. retrofit kits for them.
As per how many watts, there is not really an acurate formula for how many watts per gallon of water in my opinion. I realise that many people or sources claim that "X" amount of watts per "X" amount of gallons of water is needed but I think this type of fomula is not accurate.
If you own a coral thet needs "X" amount of Light (Lumans , LUX, PAR, etc..) with a spectral quality of "X", it really doesny matter how many gallon aquarium it is in, it needs the same amount of light and spectrum no matter what it is kept in.
Many people do a P.C. lighting system that includes a day light with between 67K to 10K with a actinic light as well. Under these type of lights, mant soft corals such as mushrooms and polyps (zoo's stars, xenia, etc..) and some LPS do ok.
There really isn't a simple "one" answer to this question. What specifically are you going to be keeping in this tank?
Joel