flex pipe size?

I was wondering if it is possible to have an incorrectly sized flex pipe? 

I took my car to a honda specialist mechanic b/c it was having engine problems and he said the remainder of my noise was because I had the wrong sized flex pipe put into the car.  He also said that over time b/c of the incorrect size it would grind and wear apart the welding. Forcing a need to rereplace it in future years.

I bought the car from a used car dealer (and have come to the conclusion that the dealer is an unfriendly full of lies manipulative person).  The dealer said it is impossible to have an improper sized flex pipe b/c the flex pipe must be fitted against the exhaust system so both the diameter and length have to be right for it to fit.  So even if the part is not a honda part it will work fine and cannot possibly cause problems.   The original was replaced b/c of extensive rusting and it was replaced by the dealer's muffler people.  The car is a 2000 honda accord (with a replaced engine b/c of a front end collision that the dealer won't admit, but its apparent from the visible body work done on the car [dealer said they replaced it b/c it knocked]). 

So should I take the car to the dealer's muffler people to ask to rereplace the flex pipe?  Is it even possible for a car to have an incorrectly sized flex pipe, and what type of problems could result from it?  And if the noise is not substantial how can I persuade the muffler people that the flex pipe they put in is wrong and they should replace it?

Thanks