Got my delete pipe from CB Engineering, although they do not specify diameter on the website the pipe diameter was 3" from turbo back to the Y pipe where it meets your stock or "Cat Back" aftermarket pipes.
I just measured to see for myself. You will see in these pics the 3" side is my stock CAT that connected to Turbo. The 2" 5/8 side is where DPF pipe met with Y adapter...I wonder if it's smaller from crimping or factory spec. idk.