Hi im just after a bit of info, i have a full jetex exhaust cat back and the back box fell off! completly rotted through

Just wondering if im right in thinking the jetex exhaust size is 2.5"?
just need to know what the standard exhaust size is on the back box where it joins the rest of the system if that makes sense lol i didnt have the jetex system fitted previous owner did and i cant find a jetex back box on its own so need to find a reducer!
Any help is great
cheers