Douglas Moore I agree with @Boudreaux, 21mm fresh collagen casings are the easiest to use and I think they have a better snap then sheep casings. If you are looking to go with a Natural Casing then you would want to look at the smallest sheep casings which are generally the 22-24mm Sheep in either the entire hank or you can get that size in a HomePack as well. The home packs will stuff somewhere in the vicinity of 15 lb of breakfast sausage. You need to rinse and soak the Sheep Casings and if you go with the Home Pack you also need to rinse the inside.
Check out this post https://meatgistics.waltonsinc.com/topic/780/sausage-casings-103-preparing-your-casings for some more information on preparing the casings before they are used.
Let us know if you need anything else!