1. The size of the area depends on the content within it as the image is a background image. To make it larger, try adding more top/bottom padding.
2. This is a bit more difficult, as the image needs to begin at the center position for the parallax to work. What happens is when you scroll, the image position slowly moves up, so changing the original position would cause some issues with the parallax effect.
3. I’m not sure what you mean? Do you have an example?
4. Parallax effects aren’t really meant for mobile, they’re more meant for larger screens with the scrolling of a mouse.