Exhaust sizing

Fellow FABOer's.

Im curious as to how one sizes the exhaust system. I bought my car with the exhaust set up, and it had me thinking. This is how its set up.

-TTI's 3" collector
-Reduced down to 2.25" right after the collector to the intake of the mufflers
-Out of the mufflers its reduced again to 2" out to the rear of the car.

Also it is not mandrel bent, so each bend is drastically smaller. Is this system sized properly, with the exception of the bends?

I know some back pressure in the exhaust is needed, but how do you determine what exhaust size should be ran.