BACKGROUND IMAGE SIZES - What resolution should they be?

as you all know having that huge big image background in your home screen is the latest fad with web design. As you can see on http://www.apple.com/ (Except some images fill the whole home page).

But I have a few questions in regards to this.

1) What resolution should the image be?

2) What file size should the background image be?

3) How do I format it so that the images don't stretch on my website? - Should I be setting a width and height as default for my webpage? - Should I be using background-size image?

4) Any other tips would be greatly appreciated.

Thanks Team :)