What the size of pipe is exhaust system on 2004 trailblazer?

The exhaust system on a 2004 Chevrolet Trailblazer isn't a single, uniform pipe size. It varies along its length. It starts larger at the manifold, reduces in size at the catalytic converter, and may reduce again before the muffler and tailpipe.

You won't find a single definitive answer for the "size" without specifying the *section* of the exhaust system you're interested in. To find the exact sizes, you'd need to:

* Consult a repair manual: A factory service manual for your specific 2004 Trailblazer (including engine size – 4.2L I6 or 5.3L V8 will make a difference) will have the precise dimensions of each section.

* Measure it yourself: If you have access to the vehicle, you can carefully measure the inside diameter of each section of the pipe with a suitable tool. This is the most accurate way to determine the size.

Without more information, it's impossible to give you a precise answer.