On the front, it's a 3.5" x 18.
On my rear, I stayed with the 16", and it's also a 3.5", and I'm running a 140/90X16 Metz. There's been a lot of discussion on the board about the rear wheel/tire sizes for the rear. You can go with a 4.25" x 18, and up to a 160 rear tire (some clearancing might need to be done on the belt guard).
This is also my OPINION, but it's based on some real facts...there is NO 18" rear tire with a load rating equal to a 16" rear. The 160/60x18 has the load rating, but is not recommended by Metzler on a touring bike, IF you ride the bike loaded up. It will wear quickly because of the excess heat generated due to lower volume of air in the tire. This is what a Metzler rep told me, and at least one person on this site can confirm this in practice. If you ride solo and not fully loaded, you can go to a 140 or 150 x 18 rear without worry, BUT if you tour on the bike two up and loaded down, I would think hard about the load rating on the 18" rear tires available...the ratings are significantly lower than the stock 16" tires. If you run fully loaded, you need a rear that is 900lbs or better, IMO.
The 18's look great on the rear, but I personally would give carefull consideration as to how you ride the bike and what you use it for. I'm two up on the Ultra 95% of the time, and tour on the bike two up, saddlebags loaded, TP full, and another bag on the luggage rack, so chose to stay with a rear tire rated for 900+ lbs.