Are these tires just not available in 190s? (rear)

Correct.  If you want a 190/60, you're looking at the Yami-recommended Dunlop and Bridgestone, or an Avon AV72.  I would recommenced the latter.

I run Avons also, front and rear! Have got as high as 13k from a set won't run any other tire! And I don't ride easy either i've worn out two crossover pipes on my duals and three set of board scrapers!

I am new with my Stratoliner but very familiar with tires on big bikes. I just put a Metzler 200/55-17 ME 888?...(the new Metz) on the Strat to lower the bike slightly...I went with Metzler because the bike has a good Metz on the front already. Otherwise I have given up on Metzler because I seem to wear them out quickly.

I went through 2 sets on Michelin Commander 2's because of how I wear my tires...otherwise it would have just been one set.  I originally tried these tires because I thought they would last longer. I did not like them because they were not predictable with varying temperatures. I don't care much for doing a 2 wheel drift on a bike that is bigger than me.  I live in British Columbia, Canada and it ain't that warm here.

My favorite so far are Avon Cobra's. I have then on a Kawasaki Vulcan Mean Streak, and when the Metzlers are done on my Stratoliner, it will get the Avon Cobras as well.

As far as the tire size question goes... You can increase, decrease or keep the same wheel diameter. Obviously a smaller diameter wheel/tire will lower the bike, if you keep the same size/diameter, no change from stock, and if you increase the diameter (aspect ratio) the bike sits higher giving you more ground (cornering) clearance. Don't forget that messing with this will affect your speedometer readings and your shift points as far as timing goes.

I have lowered the suspension on a bike and increased the aspect ratio of the tires and got a real good result for cornering clearance and a low stretched out look and low saddle height.

Always make sure your tires clear everything in the frame/ fenders with full suspension travel.

This evening I just tested out my Strat with 1.75" lowering links and a 200/55-17 Metz on the rear, stock tire size up front, and it is a bit low for performance riding but still in the ball park. I still managed 80-100mph through some twisties but I only weigh 160 lbs. I set the spring preload on the shock just a little higher that normal factory setting, but it is still a smooth ride. I am happy with it.

I hope this answers your questions and gives you some ideas.

Cheers!

Something you may want to consider in your choice of tire is how you like to ride. If you like cornering and actually lean the bike then I recommend the Cobra's. If you don't care much for that and like to put the miles down on the super slab or other highways the the Michelin Commanders

i went to dark side for rear, love it

The strat will eat back tires fast, stock bridgestones  just disappeared on me in a long trip with the bike loaded 2 up. After trying a few different brands I like avons and Michelin commander twos.