tire sizes for 18

I'm looking at going to radials sometime in the near future. What are the available choices for 18 inch rear rims? I know that 17 inch rims have more choices, but I don't like the empty space there is around the tire and the fender. I wonder about something like this...

I need to know the max size tire I can mount on my 18" rear wheel. The swingarm is extended 3" and notched. Thanks in advance.