I think you mean "reducer", that connects the collector of the header to the exhaust pipe?
In any case, the fact that it is a "reducer" is already bad, so I wouldn't reduce any more than you have to.
Ideally, expand the exhaust pipe to 1/8" bigger OD so that a reducer of the same diam as the exh pipe will slip into the expanded part. If you can't, then I'd get the same size and weld them.
