thickness of the rear one on an ls is 13mm, itr is 22mm and 800% stiffer. i have one on my car. flip the existing brackets so raised part is towards middle of the car. then it bolts right in. there's an article somewhere about it... awesome mod. my car is so flat in the corners and so responsive. do it!!
the sizes are the same on all models for the front bar. they usually come beefy because of the extra weight in the front (i.e. engine, drivetrain, people)