wheel measurements
 

I need to get a rim that matches my black "D" window wheels for a full size spare. How do I know if its 15x8 or 15x7? What about backspacing? Does it even matter for a spare?

http://jeep.us41.org/wheels/index.html
Go about half way down and it will show you what Backspacing is and how to measure and this link will answer your does it matter ? as well. If that one is not good enough than try this one.
https://www.rsracing.com/tech-wheel.html#wheeltrm

That's exactly what I needed, Thanks!