So the largest rock you'd recommend in this tumbler is fist size? Is there any chance you could use an actual measurement? My fist is over double the size of my wife's fist. We'll both be tumbling rocks. The question is for ordering the tumbler since we do not have it in front of us. Obviously after getting it we will know what size we can put in there, but for the purpose of narrowing down our shopping list, mouth opening size and maximum rock size would be great.