flex exhaust size ?

[shwnrttr]

i wanted to get some corrugated stainless flexible gas piping for an exhaust,but wasnt sure what size to get to fit tight around the stock china muffler pipe. if anybody knows what size i need i would appreciate it.
heres the stuff i was looking at http://www.ebay.com/itm/3-4-3-4inch...089?pt=LH_DefaultDomain_0&hash=item20bdb34d89