Used spacers, but it still didnt ride right, I think the biggest you should go is 19", but then again its a narrow body (long wheelbase) and larger wheels do look akward. I enjoy the 18"s theyre not too big, but not too small either. Ultimately, its up to you, and your preference.

'01 A6 2.7T Quattro
Performance: APR Stage 1 ~ APR Cat-back ~ EVO V-flow CAI ~ Vogtland sport suspension (Koni's) ~ H-sport sways front/back ~ Bridgestone Potenza RE760s 235/40ZR18 (summers) ~ Tip00032 ~ Apikol snub mount
Reliability: Samco TBB ~ Samco Upper Boost Hoses ~ Forge DV ~ Elf Synth w/ MANN ~ 93oct ~ Mintex Pads/R1 Concept Rotors ~ ATE SuperBlue ~ Timing belt/Valve covers/and plenty more (trust me).
Eye candy: 18x8 Achtuning reps ~ RS6 trunk lip spoiler ~ OEM grille =) ~ B-Raudi6 Deutsch front tag ~ Smoked side markers ~ 30% Tint ~ A6 All-season rubber mats ~ LEDs-reds/hyperwhites ~ Smoked plate cover/Quattro logo frame ~ A6 debadged- 2.7T Quattro relocated to drivers side.
Future plans: Clear Corners, Milltek DPs, x-pipe catback, AWE ICs....