I got a Cypress back in June and, at about 500 miles, the LBS recommended that I move from the stock 700x28s all the way up to 700x38s. (I'm a little "robust" weightwise, but not overly so.) I rode that way for about 1,500 miles -- no technical issues at all in making everything fit, etc. (And I found that changing tubes after a flat was mch easier; the larger tires went onto the Xero rims much more easily.) I was, however, somewhat disappointed in the ride quality, especially on longer rides (metric centuries and above), and I just switched back to 700x28s.

In my case, however, I didn't go with slicks, so I'm afraid I can't help you there.