If the letter "x" is one 16x16 tile, it means it has 16 pixels from the left to right, and 16 pixels from the top to bottom. This is one 16x16 tile.
With that said you can combine multiple 16x16 tiles to make a large tile, often called a metatile/metasprite. This could look something like the following "xx" (32x16) tile. You can comebine them to make other sizes as well. 16x32, 32x32, etc.
I'm guessing the pack you linked is talking about 16x16 tile sizes, and just considers anything large to be a metatile.
Hope this helps.