For a turbo car, the general rule is go as big as possible... that being said, you still should try to keep the car reasonably quiet. You should do just fine with 2.5" piping. (it will help keep the noise down and doesn't cost you much horsepower on a mild turbo setup). Make sure the downpipe is mandrel bent to avoid any flow restrictions right at the turbo outlet.
Just FYI, I have a full 2.5" mandrel bent turbo-back exhaust on my turbo Camry... (and a full 3" exhaust on my WRX)
2.25" is good for a N/A car with with a 2 liter displacement (for the street, in most cases). 2.25" is what most stock turbo cars come with (like my WRX). You will be missing out on some spool time and power with a 2.25" exhaust on an aftermarket turbo car... I say that you can use the TRD muffler just fine though (you will have to do custom piping to it though). As you go farther back in the car and the exhaust gasses are cooler/denser the smaller piping doesn't matter as much.