Set your desired values in Preferences > Settings   

Considerations: A value of .25 or .5 is common. This ensures if you push the crate along the floor, the dropped panels are less likely to catch on anything on the floor and be ripped off. However, you will want to weigh that against a reduction in total stacking strength.

In the Design module the Panel Drop Span value is located in the More tab. 

Note: Releases 1807nn and older will not modify the plywood dimension when the Panel Drop Span value is changed in the design. Users will need to manually change the plywood dimension when refreshing the design. This has been corrected in release after 2101nn