The rigging on my boat is 1/4. Since I am planning on doing extensive offshore with the boat I called a rigger and had him come down to look over the boat with me. He said that the 1/4 wire would be fine, no point in going any larger. However I may go a size larger and get 5/16, just for that little extra boat upside down in a storm strength. The most important thing to remember is that the wire almost never fails it’s usually always the fittings. All a larger wire really does is give you a stronger fitting.
And about the NSF I guess this is one of those things that you need to say the whole thing otherwise it means something totally different. It’s actually ANSI/NSF Standard 61. And the product is aqutapoxy. I think it may be the only one rated for 100 gallons or more. Most are for water tanks you can walk around inside of, definitely not mine.