I have just spoken to my dealership and at first they went even sure what the maximum size is. But what the mechanics said was;
1. You can fit any size you want but make sure they fit without any modification to the wheel area.
2. As long as the tyre size is correct for the alloy and its legal that’s fine.
3. He said "i don’t see why you fit 16" or even 17" as long as the tyre is appropriate for that alloy size.
4. Let your alloy specialist recommend the best and safest size for you.
Personally I will not be getting any size bigger than 16". but if the ride becomes uncomfortable I will reduce to 15".
Remember just get the correct tyre size for your alloy.
here are some alloyz i'm considering. in the uk the best place to get them is at