205/50-16 rides very smoothly on the xB with stock suspension, and there are a TON of tire choices in that size. FYI-
185/60-15 = 23.7in stock
205/50-16 = 24.1in what I'm running, makes speedo match GPS perfectly
215/50-16 = 24.4in about the limit of the wheel well without rubbing
215/35-19 = 24.9in what came on the car from lot- rubbed at full lock in front
Just remember that suspension setup and travel have a lot to do with ride quality. The xB has quite a bit of wheel travel range in stock form, so it rides smoothly even on 205/50 tires. 215/50 would have been my choice- but it was more expensive and I didn't want to take a chance on clearance or going too large.
225/50-15 is a SWEET setup for tons of grip and a nice ride on 15x8 wheels that puts you just a hair over the stock tire height @ 23.85in, but the tire selection in that size is a little slim. I would have run this on Katie's car if I was able to find cheap 15x8 wheels in the right offset.