How to set background height:

To set background height of the zone, first click on design mode. Next, click the background tab and choose a zone. Then, hover over the zone and click the pencil icon. After, adjust the height with the slider or type in the pixels.

Here is how to set background height:

Step 1: Click the design tab. 

Step 2: Click Background tab (A). 

Step 3: Choose a zone (B). 

Step 4: Hover over the zone and click the pencil icon (same as editing a widget)

Step 5: Adjust the height with the slider or type in the pixels (C).