You can easily do that for that particular menu item, follow these steps to make the changes:
Go to that Menu item edit options
Under Astroid Menu Options tab, you will see the Width and Dropdown Alignment options.
Set the width of the dropdown menu under Width option & if you want it to be the size of the main menu then choose the dropdown Alignment to be Full Width.
(See the screenshot for reference: www.screencast.com/t/QVdbhdFb)
Let us know if this worked or you have any other issue!
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