How to reduce mega menu width in Hestia

To reduce the width of the mega menu in Hestia start by going to Appearance->Customize->Additional CSS and add the following code:

.navbar .hestia-mega-menu > .dropdown-menu {
    width: 800px !important;
}

Change the 800 to a lower value to further reduce the menu width.

That's it! Publish your changes and you're done.