looknig to get a new backbox just to compliment the RS replica look ,and wondered what size the pipework is?

on my subaru i had a full 3" decat system, not even the backbox on this is 3".

is the pipework 1.5" ?? and does it vary from a 1.4 to a 1.8 or 1.0

want to get a new backbox but dont know the size pipe i need to order/ ebay. car is a 1.4

cheers