your basic idea is right, however the details are wroung
1.there is no size limit "however if u have a 5inch exhaust im guessing ur legal ride height is compromissed"
2.you can have your exhaust tested at a exhaust shop, however it wont do shit for you when the police cheak it
3.the police cannot send you to the botany or penrith testing station's, they can however notify the d.e.c.c "e.p.a" which inturn will send off a nice lil letter, requesting you pay them a visit and "cheak" if you have all legel/certified modification's. also note an engineer's for the exhaust,fmic mean's jackshit because the testing station cannot certfy that is the exact same exhaust,cooler etc etc.. therefore if its not standard chance's are you will asked to rectify these problem's or head down and have it pass an emission's test..
so in short, there is nothing you can carry around with you to certify that your car is legal, from a noise side or emission's. engineer's etc only help's if the coppa doesn't do his job.