If you want to make your drop down menu to full width then you have to change menu code.
Because drop down menu open under main menu and its width defined less so you have to change most of code and its called mega menu. For that you must have knowledge of liquid,html and css, Let me know if you need help.
That sounds wonderful! I'm happy you found a solution.
If you have any other questions please don't hesitate to let me know!