Unbleached cotton muslin.
To find the width for for a particular diameter.
For a 2" diameter casings.
2 X (pi)= circumference.
2 X 3.14= 6.28
6.28" is the circumference.
3.14" is the width when folded, but you must include extra material for the seam (seam allowance).
You don't want an interior seam that'll get stuck in the sausage.
A serger sewing machine is best, but, of course, most folks don't have a serger.
So, alternately, you can sew what's called a "French seam" with a regular old sewing machine.
The following video shows how to sew a French seam, but she sews it on the inside, you'll need to do the opposite and sew it on the outside.
[VIDEO][/VIDEO]
HTH
~Martin