can 16x16 be bigger?
That depends entirely on what “16x16” is actually counting. My guess is that in this case they’re using that size for their tile grid and designing some characters to be bigger than that. It’s just badly explained on that package.
#x# should always correspond to the size of what you’re measuring, whether that’s the tiles, the characters, everything, etc. It’s up to you to decide what to measure though, so anything else can be whatever size you want as long as it makes sense.
I don’t know of any videos explaining character size and resolution options myself, so the rule of thumb I’d give you is to look to old game console sprite and tile limits for inspiration. That (along with computers liking powers of 2) is where a lot of classic pixel art metrics came from.