You can usually go up about 1 inch in overall tire diameter on stock suspension without rubbing at full lock. The RS6, for example, runs 235/65R17 from the factory, and owners have fit 235/70R17 or 245/65R17 without issues on most trims. Anything beyond that and you're looking at fender contact on bumps or tight turns. A mild lift opens up more room, but we won't prescribe exact sizes here. Check your specific trim's fender clearance before committing.