what are the best and biggest brakes you can pop on an A4 with the stock 16's? and is it different with 18x8 wheels?

 

for the front the pistons can just be compressed but for the rear they need to be rotated while compressed. so the special tool is one that rotates and compresses at the same time.

but the hardest part will probably be getting the roters off the hub. they have a tendency to stick on there, just sledge hammer them off.

and yes wheel size is important when choosing brakes. some will be too big to fit in a 16 or even 17 inch wheel