A quick look at Bridgestone's site shows the following rim sizes for a 190/50ZR17 tire, all types:
Suggested rim size 6.0
Acceptable rim size 5.5-6.0
The B-12 has a 5.0 inch rear rim (at least according to our Wiki), so well under the recommended size. That being said, the factory fitment is a 180/55ZR17, which Bridgestone recommends a 5.5 inch rim for as well. Usually you can go up or down 1 size from factory, but it looks like Suzuki has already pushed the limit.
Remember, this is Bridgestone's list, other tire manufacturers may have different recommendations.
Personally, I like the handling of a narrower tire...the little 600 only has a 160 rear, in basically the same frame layout as the bigger 1200, and this is part of the reason that the 600 turns so much easier (other reasons being less engine flywheel/mass, smaller front rotors, less weight, etc)