Id never go smaller than 18s on a Mk4 TBH, they have huuuge arches, even 19s dont look remotley wrong.

19inch RS8 wheels always looks good, and the replicas are identical looking and pretty cheap.

Make sure you get wide wheels, and a decent offset, dont want to bring the gay cossie trend of skinny wheels about a mile inside the arch into the VW world.

IMO id stop asking the least styled cars in the modding scene, and go join the obsessives on edition38 and ask them, they might be way OTT in their love of VWs, but they got taste, and know the answers.

(and wont tell you to buy wank like wolfraces )