Single din with a big screen means the screen folds out and kinda sits there. Where as double din sits flush with your dash and the screen doesn't move (unless you get a model where it moves to put a CD in). I don't like flip out single din screens.
Crutchfield has documentation to wire the harness into your car. It's pretty good. I'd be willing to help on a weeknight since I work near you in portland, but I'm not the best at wiring and provide no warranty expressed or implied. I'd just be looking at the directions and going from there.
Here are some examples...
This is a wrx with a single din stereo. The extra space is filled by a cubby pocket:
This is a single din flip out GPS stereo. They are dumb:
I'm assuming you have the stock 6 disker. It is a double din, note how it fills the whole dash area: