I'm looking for a twin exhaust/backbox similar to the stock ones on the 325/328 but after scanning through the internet can not find the size of the tail pipes!

I have found a few after market ones, but they all seem to be 3'' on each pipe which I am thinking sounds like it could be too big for the recessed bit in my bumper.

Could anyone enlighten me on what the stock size for the twin pipes is and if anyone has even seen a similar part fitted to a 318/316?