Originally Posted by ptzdodge
I also recommend that if you are planning on ordering the parts prior to removing the old parts that you measure the size of the rotors since there can be several different sizes (measured in mm) for your car's rotors (this is a fact I learned the hard way). Also, the pads can also be several difference models so in all I recommend that you remove the parts prior to ordering the replacements to ensure that you receive the exact size you need so it reduces your frustrations and trips to the auto parts store.
Also, I recommend that you have on hand a breaker bar, C-clamp to push the piston back into the Caliper (unless you invest about $65 in a brake tool to do this) and a metal coat hanger (I use it to hold up the caliper so it's not hanging by the rubber brake hose).
While checking never hurts, all B6 A4 1.8T rotors are the same size. The caliper bolts are 21mm (13/16") and a 13mm wrench takes them apart.
You need to get the caliper tool, that can be rented for free at Advanced/Autozone. Obviously some other tools might be needed but those are the basics.