So i have the stock 17's. Planning on going aftermarket to 17 or 18. Tire size though? Stock is 205. How wide can I do and be safe? Is 225 too wide? Are spacers available for Mini's giving me more room to go up even more? Thanks in advance

 

215's seem to be the largest trouble free fit. Spacing out wider would make things more likely to rub.

 

If you use 17" wheels then-

215/45-17 is OK but if you have lowered suspension then you will have less clearance

225/45-17 is too tall depending on which year MINI and how wide the rim is. Greater risk of rubbing.

In 18" wheels-
205/40-18
215/35-18

Wheel spacers are for giving more clearance from the brakes or inner suspension or for pushing the wheel outward for a flush look. If you do not need spacers that would be better so choose wheels with the correct offset. Spacers usually are not much help when using wider than OEM tires.