awkThe size is for the outside diameter. The walls are approximately 1/2" thick so if the outside diameter is 6" the inside will be 5".
The bigger sizes are great for large scale food prep, or if you just want to grind enough to fill a large shaker. In most cases you want to grind while prepping the meal so the oils and full flavor are retained. If you are cooking for just yourself or a small family the 4" - 6.5" sizes will provide ample room to get the job done.