I have two bikes in my garage right now ... my new RIII Roadster and an '07 M109. The M109 was mentioned earlier in this thread ... here is what I have for my back tire: 250/40 R18. The stock is a 240, but a 250 fits on the stock rim just fine. And I admit to really liking those 18" rims. Wide and low profile ... looks really good.
It happens to be what I would love to put on my RIII. 250 w/ an 18" rim. Or dark side, perhaps, on an 18".
If anyone is curious about other M109 stuff, I have both parked side by side until the 109 is bought.