Outer Limits 4x4 Board

Im going to get my exhaust done from the turbo back..
Guy at the shop reccomends a 2.5 mandrel bend from the turbo back.

WHat is more desireable?
it's a 3l turbo diesel.

What diiferences are there between the 1/2 inch.

Thanks for your help

3" is always going to be better than a 2.5"

volumes of pipes are like this if i remember physics/maths correctly

a 2" will hold 4 times the volume of a 1" etc etc

i should have been more specific.

It's a Gu patrol turbo diesel. 2001 model.

I thought the theory with exhaust is to create some sort of even back pressure and not simply go for the larger diameter.