I think 18" is a nice size. I run 17's... done again I would do 18's.

I was able to borrow a test wheel and tire for fit measurements then ordered wheels with custom backspace (American Racing). If you are trying to fit the biggest rubber, that helps. Most aftermarket wheel manufacturers will make custom backspace to order.

Once you get close on a tire size and type, look at what is available. I used Tire Rack to explore options. More common sizes have several options which helps with cost.