Mann4ducks is right on, kinda like common wire sizes, the smaller the number the larger the fly size. The size of the fly is actually just stating what the hook size is. For example, a #14 fly is a fly tied onto a #14 hook. Obviously, the larger the hook the larger effective fly size is since you don’t want a big chunk of metal/hook shank sticking out.
I wouldn’t worry too much about the largest or smallest. Depending on what you’re targeting, I’d mainly stick to basics and get the same fly in a couple sizes on either side so you’re covered. The best way to judge what will work is to match the hatch- see what bugs are on the water or below and try to mimic their size and style. Also, don’t give fish too much credit, sure some giants are picky but for the most part it’s like floating a doughnut in front of a fat guy- chances are they’re gonna eat! If you’re in the Midwest, don’t look past using them for warm water species too. Panfish and bass are a blast and readily bite.