It depends on which one you bought. They have 3 different options: 2.25", 2.5", and 3.0". I bought the stainless steel 2.5" exhaust with the Magnaflow muffler. I also have the zzp downpipe and midpipe and I must say the sound that little 1.4 makes is quite nice but it is LOUD! My main issue with this kit is where the muffler sits. Instead of using the space where the stock resonator or muffler goes, this puts the muffler in the middle of the car. My car is lowered by about 2 inches and the muffler scrapes on everything! When you look under the car you can see the big old muffler hanging down below the rest of the car by about 2 inches. Even if you are not lowered you are guaranteed to scrape on certain bumps and speed bumps.
I wanted to give a little more insight for people who are looking into this kit. My driver side rear suspension hits the exhaust when I hit big bumps on the road and is quite annoying. The last thing and I think the most important is diameter. If you are like me and purchased the zzp down and midpipe, then without question get the 2.25 inch. The reason I say that is because the zzp down and midpip are 2.25inches. You are not going to see additional power gains with a larger exhaust diameter that far away from the turbo like what they had told me. They just said the larger the diameter the more power I will make which in this situation is not true. I only wish I had measured the diameter of ZZP before buying the 2.5inch.