Im looking to get a Magnaflow muffler so i need to know my stock piping size.

 

I think its 1.75".

 

Yep, stock LS and LT cat back exhaust has an outer diameter of 1.75"

 

Yup, 1.75"

And don't get anything larger than 2.25" please...

 

what will 2.5" piping do adversely that 2.25" is OK for?

(forgive me, I come from a world of much larger engines thus larger piping LOL)

 

It's the effects of exhaust velocity and the negative effects caused by it. It will basically slow down the velocity build up and scavenging effect to where the powerband will actually get shifted upwards. Might experience a loss in torque in the mid-range but a gain in horsepower in the peak. A 2.25" piping setup would be a more suitable exhaust size to support MOST of the powerband and not a narrow portion of it. If he was more highly modified, my answer would be different but cause of the engine type (very low RPM related, torquey and not peaky in HP in higher RPMs...mainly characteristics of the stock camshafts) I say 2.25 is good enough.