Big Country:
That's a tough question by itself. There are a lot of factors to consider. The SSR has stock 20"x10" rear wheels as we all know. I'm pretty sure 11" wide wheels would work, and if I'm not mistaken I believe Fanatic "pistons #1 fan" has 22"x12" wide custom built rear wheels on his SSR. The back space would be the real concern, so not to interfere with the brake calipers.
I'm not sure why anyone would want to consider anything bigger than 22" for the rear wheel diameter. By the time you get a tire that has any sidewall height to it, you will wind up very much oversized in total O.D., way over the 3% recommended.
A P295/35R22 size tire (if available in brand you want) would result in a tire O.D. of 30.13", or about 2.9% oversize compared to the 29.29" stock tire O.D. Remember that most custom cast aluminum chrome wheels that will fit the SSR are 20"x9" and 22"x9.5", so most people will tell you not to use a tire wide greater than P285 for the 9.5" wide wheel. Custom polished billet aluminum wheels can be found, in 10" widths, and wider, I believe.
You might want to check out the thread I started several days ago on custom wheels. Here's the link:
Custom Chrome Wheels - "20/22 Look"
Hope this helps.