Dang, that sucks. You can actually measure for rims pretty easily and there are lots of tutorials out there, whether you'd rather read about it or see it visually on Youtube. Pretty much it comes down to the space you have inside the wheel opening. You could put a crazy negative offset wheel on it that would stick out several inches, but it would have to be a tiny diameter or it would interfere with turning and hit when you went over bumps. Similarly, if you wanted a 17" rim you'd have to pick something that's narrow enough and has a higher offset to fit inside the fender. That's a rough generalization of course.

Sounds like you'll just want a 2mm spacer, as long as they're not too wide. Measure first to be sure. The rears are a lot more forgiving for sizing but you want it to look even anyway.

If you run a Diesel Purge you can put monster truck tires on it. A little tip for ya.