Just like sandpaper, the smaller the grit size, the larger the grain size. An 80-grit sheet of sandpaper has much larger grain size than a 220-grit sheet.
Most dealers carry grain sizes ranging from 30 grit to 90 grit (on average) and all of the grit sizes are a-okay for making high-temp mortar! We’ve built ovens with 30 grit and ovens with 90 grit and it’s hard to see a difference between the grain sizes with the naked eye when the 4 ingredients are all mixed together!