You can modify the CSS that works for different sizes of device.
Under theme.css you can find css mentioned with width of the menu and then modify the CSS.
Could you help me specify which code work for which platform? Basically, I am unsure how to edit for laptop. I found the mobile code but not for laptop.
I have checked your website menu. The drop down menu width will be equals to the main menu width on all screen sizes, as you reduce the screen size the menu width also decreases and the sub menu width fits inside the main menu width. I am sharing a code with you, please add this in assets → theme.css file at end of the file.