[SOLVED] Page Menu Sub Menus of Equal Width

I've done several searches and not found the answer to this question.

What code do I enter to make the drop down sub menus default to the width of the pages in the Page Menu Bar? The control panel under Page Menu Sub Menu only addresses specific pixel widths but not how to code the "em."

Where do I find the proper page to enter this code?

Thank you for your time and assistance.