hi zanarc. and welcome.
first, what i would do is read some great articles that are provided up above. that will explaine alot of what it takes to care for any SW
aquarium. their great god knows i have used them alot when i first started. lol and second i would try to go as large as your buget would provide. due to the fact that a larger tank is easier to care for then a smaller one.
and it would also depend on what u are looking to keep. just fish or fish only with live rock or mabey even a reef set up. i have see a 1/2 gallon reef set-up ( crazy ) lol
but i am no pro at SW
yet but i am quickly learning and i am loving every day of it. but if u were just thinking of Fish i would have to say u could keep 1 or mabey 2 very small fish. mabey a clown (nemo) and some other small one like a globy. their both very hardy and great to learn with.