Personally, I think 6 x 14 wheels are big enough - ideally fitted with 185/50 tyres.
You can fit wider wheels, but the G40 is very light and the front suspension layout is what limits grip as opposed to wheel/tyre size.
Wheel offset is very important too, ET38 to 33 is ideal. It really doesn't help having the wheels sticking out too far, because it increases tramlining, the effect of torque steer, tyres scrubbing arches and more load/wear on wheel bearings etc etc.
I'd keep away from ultra low profile/very stiff sidewall tyres stretched onto wide wheels too, the lightweight Polo will just skip about on them on anything other than perfectly smooth roads (do they even exist?!)
I still think there is a lot to be said for the standard 5 1/2 x 13 wheel when fitted with decent quality 175/60/13 tyres.