I would say 8" is optimum for me but I admit I like bigger cocks too! This is me and a friend taken one night a few years ago when we were camming. The first time I rode him and let that monster slide balls deep inside me I thought he would split me like a log!
Once I relaxed and got used to his BBC it was an incredible experience! Preparing yourself for that size cock makes all the difference. In my humble opinion an anal trainer set of three butt plugs sized small, medium and large and a lot of water based lubricant like Astoglide is a must.