Don't Trust This Chart
Your thin-set probably has a chart like this on the label. Don't rely on it. The recommendations are a good starting point, but they don't guarantee a thin-set bed thick enough to provide full contact with the tile. And without full contact, you don't get full support or adhesion.
As the chart shows, larger tiles require larger trowel notches (to provide a thicker bed). But other factors matter too: the flatness of the wall or floor, or the texture of the tile's back. So the only reliable way to know that the bed is thick enough is to set the first few tiles, then immediately pry them up. If the tile hasn't made full contact, you'll see it. The easiest solution is to use the next notch size. With tiles larger than 12 in., it's a good idea to also “back butter” them with thin-set. Also keep an eye on “squeeze-out” during the job. If you don't see thin-set squeezing out between tiles, pull up a tile to check coverage.