I did a little tire shopping yesterday and made some observations...

For my 2009 stock size recommendations from the owner and shop manuals:

front    100/90-19
rear     140/80-15

A lot of tire sites will lure you in with tires "starting at $80" or some such, but rest assured, your particular tire is guaranteed to cost more.

While the front tire size is plentiful, very few of the manufacturers make rear tires in that size (surprise surprise!).   However, tires with an aspect ratio of 90 are more readily available.   Those tires would add about 28mm to the tire diameter (25.4mm = 1 inch).   I have read some threads in here which seem to indicate the increased diameter is still OK.   Keep in mind this works out to about a 4 .. 5% drop in engine RPM due to the increase in tire diameter, and a corresponding change in speedo reading (you'll actually be going faster than indicated).

I measured my stock tire (rear) and as close as I can tell it's about 24 inches in diameter (but perhaps half worn out).   Does anyone know if we can safely say that 90 aspect tires are OK as long as the width is kept as 140mm, or is it tire dependent?   I'm wondering because some tires may have more tread thickness.   Also I'd be curious about the largest tire size that can be used (width and height) without rubbing on the belt, swing arm, fender, etc.?  This may have been answered already, and if so, I apologize for the redundancy.

I just bought a new set of Pirelli Route 66's and put on my bike and the 140/90 fit just fine.  No rubbing anywhere.