I guess it is a 17" on the rear at least.
I fitted a Metzeler MeZ 2 on the rear, simply because it was the only tire available in an isolated area. OEM on Thruxtons and Bonnevilles. Have liked it. It's a steel belted radial and makes the rear feel nicely planted. Up until then I have used the Sport Demons (OEM when I bought it) and they have been pretty good. However, as KiwiDave has said, they wear pretty quick on the rear. The front I usually get about 12,000 miles.