You could go with the small tank if you wanted. It will be louder because it will be oil-less and will fill more frequently, but it would be very portable. One reason why this would work well is that finish nails won't be shot as frequently as framing or roofing nails so the tank filling more often shouldn't be too much of a problem. A 3 gal like Mr Ratchet suggested, hand portable but still oil-less, would work well too. I have an oiled 8 gal that I like. Not as portable with it being heavier and on wheels (I can still pick it up tho). One very important thing is to have plenty of flexy hose for a trim gun, I have 50' of 3/8" goodyear hose plus 50' of 1/4" flexzilla.
I know I see a lot of the 6 gal oil-less pancakes.