I have a box. I want it to be 100% of the width minus a couple pixels. I also want to add padding inside the box, without making the whole thing bigger.

How?

Padding is not INSIDE the box at all, it's added OUTSIDE the box -
http://www.redmelon.net/tstme/box_model/

I want it to be 100% of the width minus a couple pixels

Soo... don't set a width, it will be 100% by default, and give it a couple of pixels worth of margin.