You might start by reading this article which talks about aspect ratios:
Basics of Digital Camera Pixels
If you scroll down just past the half way point in that article, you should see a graphical representation of how aspect ratios work.
And my guess for why "image size" isn't available in RAW is because it's the untouched image data. It's not reducing it's quality or size because you don't want the camera to do anything with it. I'm not sure if any cameras let you change image size when you're shooting RAW (though I don't know). I do know that mine doesn't.