This setting is available if the Label Orientation is set to Vertical or 45 degrees. This property determines the height of the area that’s dedicated for x-axis labels. Enter your desired amount in pixels (ranging from 20 to 300px), and the visual will move the chart higher to make room for category labels. 

For Vertical label orientation setting, this property is governed by Axis Labels Expand Height if enabled. Disabling that toggle will allow you to manually force your desired axis height.