I would go with the widest tire I was comfortable with and could get the best deal on. You will have the most selection with 235/35r19s if you wanted to stick with 19's which will look good, including a pretty decent selection of all season tires as well as performance summer tires. 18's are generally going to be cheaper and lighter and you will have a much bigger selection there as well (235/40r18s). I wouldn't go with anything smaller than 18's, but that may just be me.
Most of the xi's that we see and service in our shop also run spacers on the rear for the staggered look and the benefits of a wider track. It is also beneficial so that you can maintain that look when putting snow wheels and tires on.