Wow, I wonder why I was never notified of this topic – my apologies!
The box type is the outer container of the content.
The inner box type is the inner container of the content.
So you can have the box type (outer container) set to fluid (full width) and the inner box type (inside container) set to contained (the width you have set in “Customize > Layout”).
The image size really depends and requires some playing around to get working correctly – it should however be larger than the area it is supposed to show up in as it has to move with the parallax option.
Let me know if you need more information – sorry again for not catching this the first time around!