Why do you use the Stretch row option for the menu static block http://prntscr.com/q9yiwh ?
It stretches the row out of the menu columns width that you set in menu settings http://prntscr.com/q9ylq5 . If you use Static block option to show mega menu that option (columns) just limits the mega menu block width.
Watch the video tutorial how to create mega menu using static block https://youtu.be/f07MBdnFPfk
