How to Change Size of Hover Image in Nav Menu?

How do I go about making the boxes that show up when you hover over the navigation menu items smaller on this site?

I selected that portion of the screen and inspected it from within Firebug, but I’m not sure which value modifies the size of the boxes upon hover-over.

I’ve spent some time changing some values in Firebug, but have unsuccessful in identifying what specific value modifies this.