up to you. Personally I always felt that the B7 looked best with 18" wheels. Maybe that was to help me not lower the car lol. On that note, 19s look kinda silly on stock ride height…not sure if you planned to stay stock.
The RS4 looks just right wtih 19" wheels, but it accomplishes this with the aid of massive flared fenders and a 1" lower ride height. The B7 S4 can look a little big-wheelish with 19s. The DTM for whatever reason manages the 19s a bit better but again, a ride height drop is really paramount to this being succesful.
this is cool…great comparison
B7 A4 lowered (top) with VMR708 in Avus silver (must be the only one on earth) vs. my B7 Rs4 in Avus Silver (one of 2000 on earth lol)
http://www.audizine.com/gallery/data/500/medium/0426000644.jpg
http://audirevolution.net/addons/albums/images/889728883.jpg