Why don't you just go measure it. It's pretty easy if you have a set of calipers.
If not just use the string trick. Wrap a piece of string around the exhaust pipe. mark where it comes around and measure that.
Now you have your circumference.
d = ( sqrt( C / pie ) ) * 2
there you go that's the ouside diameter of the piping.
I would do if for you but I already changed my exhaust. Maybe somebody has a set answer ?
Anyways if your getting a custom exhaust just ask the shop.