The basics of wire sizing is so very simple, please have a look in your friendly West Marine
catalog. There you will find some nice colourful charts
which answer your questions.
But, for the application you have, with the long runs supplying o nly LED fixtures, there is no problem. The smallest wire that would be physically strong enough will be electrically sufficient. In this case, IMO, that means #16. For the spreader lights it depends on just what you have there. Do your own check... find out what the current
for those lights will be, estimate the length of the run (and remember that it goes both ways, so the length of wire is twice the distance up the mast
plus whatever there is below decks) and use the chart to determine minimum wire size.
But really, mate, the wire weights that you are dealing with are such a tiny, tiny fraction of the total weight aloft that it simply does not matter. IF it were me, I would be thinking that using wire that is somewhat over spec might help avoid having to replace it should some pesky corrosion
come about! I am absolutely certain the the few ounces of extra copper will make NO difference in your boats performance.