Okay, so your car came equipped with 16" rims/tires.
Remember - to a large degree, Audi engineers match suspension capability to tire sizes, so, all things being equal, simply putting 18" or 19" rims on your stock-equipped car's suspension will likely put you in an imbalanced state....certainly liveable, but not ideal.
If you plan on upgrading your suspension, then "never mind"...
Add to that the threat of rim damage with 18" or 19" rims/tires, and you can see where I'm going.
I have a stock A6 4.2. It came with the sport suspension, so it has stock 17" rims. Some fellow forum guys are running 19" rims/tires on their 4.2's and LOVE it. As for me, I've decided to put the BEST 17" tires under me, and pocket the difference in cost for my swaybar kit (already installed), and hopefully some subtle body cosmetic changes.
I've owned cars with 18" rims/tires, and it was a noticeably harder/rougher ride. Nevertheless, I "lived with it", although at times, it was a little tiring.
It's all "what you like"....just remember, if you go with 18's or 19's, be sure to run HIGH air pressure (perhaps as high as the tire can take) to prevent potential rim damage in case you encounter the many potholes out there..
Happy motoring!
Shoman