The absolute maximum for winter use would be an 18" wheel, at either 7.5" or 8" wide, with a proper winter tyre on...they will be good, but not as good as dropping to a 17" wheel, with a narrower, taller sidewall, proper winter tyre.
Or if you are really serious, the 16" winter sizes suggested by VW.
16" fit over the brakes just fine...but if you are planning bigger brakes in the future you'll probably need to stick to 17 or 18" wheels to clear them...
The smaller the wheel, the more protection for the suspension and the narrower the tyre you'll find available.
Stick to 18" and you'll have to stick to a 225/40/18 pukka winter tyre.
Due to potential brake clearance issues in the future, I've gone with BBS CH in 18" with Vredestein Wintrac Extremes on..all ready for November when I'll put them on for 4-5 months.
