I've got an 18"/21" combo coming for my lo........I agree with the 21" front on that bike, going to an 18" would look great, whether your going 200 or not, I do think the biggest you can stuff into the rear fender is a 160.
For reference, it would be a 16x5.5, 17x6.25, or an 18x5.5. With the 17 the tire will be stretched a bit wider as the rim is 3/4 of an inch wider than the 16 or 18. As far as sizes that we sell for rear wheels most often, it would have to be the 17x6.25. Metzeler makes a 200/50/17 that people love and has a nice low sidewall. On 5.5" rims there are a couple of tires available as well as the 18.
For reference, it would be a 16x5.5, 17x6.25, or an 18x5.5. With the 17 the tire will be stretched a bit wider as the rim is 3/4 of an inch wider than the 16 or 18. As far as sizes that we sell for rear wheels most often, it would have to be the 17x6.25. Metzeler makes a 200/50/17 that people love and has a nice low sidewall. On 5.5" rims there are a couple of tires available as well as the 18.
Good info!
If you go 18", you really want to slam it, because it looks (IMHO) awful at stock height with a big ol' fender sitting on top like a German helmet.