I just saw Dick Sing demonstrate turning the 4" pepper mill which did not require the use of extensions for the Forstner bit. His comment when someone asked why make a small 4" mill -- 'how much pepper do you need before you need to refill with fresh pepper?'
I've made a 6" mill, and for home use, pepper lasts a long time. You can still get a nice shape on the 4" and 6", but longer ones allow you to play a little more with shapes, but you may need the Forstner bit extension. Shorter ones will get you started to see if you like making them.
Unless you know you're going to make a bunch of mills, I'd start with less expensive Forstners. Colt makes a good bit, and has a neat MT adaptor that fits in your tailstock instead of using a Jacobs chuck.