On my largest breakpoint I want my background image size to be set to full. But, I would like to set this size to a smaller size as I go down the breakpoints. Does this happen automatically, or do I need to set the background image for each breakpoint and then select the size? Why does the option to set the size disappear on other breakpoints? Is this a bug?
The quick answer is that you need to set bg images per breakpoint if you want a different size/orientation.
If you set a bg image on your base breakpoint, it applies all other breakpoints (larger and smaller) unless you specific a bg image at a diff breakpoint.
xxl base l m s
a bg image set on base applies to all breakpoints. If you set a bg image on m or xxl, then that bg image will display when the viewport is the size of the breakpoint.