The limitations are maximum rotor width (1.25"), the depth in the wheel for caliper clearance, and how much you want to spend on a custom hat. Our "stock" hat has only a 8 x 7.0" pattern for the rotor attachment: too small for any 14" rotors that I could find. The biggest rotor that I could find to fit that pattern was 12.72". A custom hat might be $500+ for the pair.
Some 17" notes
My guess is that the selection of 17" tires is better than 18". The maximum widths (tires and wheels) mentioned in the link above are... the maximum you can put on without rubbing.